In Bento4 1.6.0-638, there is a null pointer reference in the function AP4_DescriptorListInspector::Action function in Ap4Descriptor.h:124 , as demonstrated by GPAC. This can cause a denial of service (DOS).
How to reproduce:
```
1.check out latest code, 5922ba762a
2.compile with asan, 
    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address  -g")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address  -g")
3.run ./mp4dump --verbosity 3 --format text  poc
```

You can see the asan information below:

```

=================================================================
==633802==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000003c3e48 bp 0x7ffcbc9d4550 sp 0x7ffcbc9d4470 T0)
==633802==The signal is caused by a READ memory access.
==633802==Hint: address points to the zero page.
    #0 0x3c3e48 in AP4_DescriptorListInspector::Action(AP4_Descriptor*) const /home/lly/pro/Bento4/Source/C++/Core/Ap4Descriptor.h:124:21
    #1 0x40bdc2 in AP4_List<AP4_Descriptor>::Apply(AP4_List<AP4_Descriptor>::Item::Operator const&) const /home/lly/pro/Bento4/Source/C++/Core/Ap4List.h:353:12
    #2 0x40bdc2 in AP4_InitialObjectDescriptor::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4ObjectDescriptor.cpp:327:22
    #3 0x3e0485 in AP4_IodsAtom::InspectFields(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4IodsAtom.cpp:112:29
    #4 0x37117e in AP4_Atom::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.cpp:263:5
    #5 0x39f0a2 in AP4_AtomListInspector::Action(AP4_Atom*) const /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.h:601:15
    #6 0x39d3b1 in AP4_List<AP4_Atom>::Apply(AP4_List<AP4_Atom>::Item::Operator const&) const /home/lly/pro/Bento4/Source/C++/Core/Ap4List.h:353:12
    #7 0x39d3b1 in AP4_ContainerAtom::InspectChildren(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4ContainerAtom.cpp:220:16
    #8 0x37117e in AP4_Atom::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.cpp:263:5
    #9 0x359b43 in main /home/lly/pro/Bento4/Source/C++/Apps/Mp4Dump/Mp4Dump.cpp:350:15
    #10 0x7f899655d0b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #11 0x2a2b1d in _start (/home/lly/pro/Bento4/cmakebuild/mp4dump+0x2a2b1d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/lly/pro/Bento4/Source/C++/Core/Ap4Descriptor.h:124:21 in AP4_DescriptorListInspector::Action(AP4_Descriptor*) const
==633802==ABORTING

```

[poc.zip](https://github.com/axiomatic-systems/Bento4/files/7043072/poc.zip)



Null pointer reference in Ap4Descriptor.h:124
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : Null pointer reference in Ap4Descriptor.h:124
Issue date:

--- start body ---
How to reproduce:
```
1.check out latest code, 5922ba762a
2.compile with asan, 
    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address  -g")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address  -g")
3.run ./mp4dump --verbosity 3 --format text  poc
```

You can see the asan information below:

```

=================================================================
==633802==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000003c3e48 bp 0x7ffcbc9d4550 sp 0x7ffcbc9d4470 T0)
==633802==The signal is caused by a READ memory access.
==633802==Hint: address points to the zero page.
    #0 0x3c3e48 in AP4_DescriptorListInspector::Action(AP4_Descriptor*) const /home/lly/pro/Bento4/Source/C++/Core/Ap4Descriptor.h:124:21
    #1 0x40bdc2 in AP4_List<AP4_Descriptor>::Apply(AP4_List<AP4_Descriptor>::Item::Operator const&) const /home/lly/pro/Bento4/Source/C++/Core/Ap4List.h:353:12
    #2 0x40bdc2 in AP4_InitialObjectDescriptor::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4ObjectDescriptor.cpp:327:22
    #3 0x3e0485 in AP4_IodsAtom::InspectFields(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4IodsAtom.cpp:112:29
    #4 0x37117e in AP4_Atom::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.cpp:263:5
    #5 0x39f0a2 in AP4_AtomListInspector::Action(AP4_Atom*) const /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.h:601:15
    #6 0x39d3b1 in AP4_List<AP4_Atom>::Apply(AP4_List<AP4_Atom>::Item::Operator const&) const /home/lly/pro/Bento4/Source/C++/Core/Ap4List.h:353:12
    #7 0x39d3b1 in AP4_ContainerAtom::InspectChildren(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4ContainerAtom.cpp:220:16
    #8 0x37117e in AP4_Atom::Inspect(AP4_AtomInspector&) /home/lly/pro/Bento4/Source/C++/Core/Ap4Atom.cpp:263:5
    #9 0x359b43 in main /home/lly/pro/Bento4/Source/C++/Apps/Mp4Dump/Mp4Dump.cpp:350:15
    #10 0x7f899655d0b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #11 0x2a2b1d in _start (/home/lly/pro/Bento4/cmakebuild/mp4dump+0x2a2b1d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/lly/pro/Bento4/Source/C++/Core/Ap4Descriptor.h:124:21 in AP4_DescriptorListInspector::Action(AP4_Descriptor*) const
==633802==ABORTING

```

[poc.zip](https://github.com/axiomatic-systems/Bento4/files/7043072/poc.zip)



--- end body ---
