There is issue in block table parsing in preR13
Example: 
[BLOCK1.DXF.gz](https://github.com/LibreDWG/libredwg/files/8549851/BLOCK1.DXF.gz)
[BLOCK1.DWG.gz](https://github.com/LibreDWG/libredwg/files/8549852/BLOCK1.DWG.gz)

Actual parsing:
```
...
blocks   0x46c (0xfffffb94) - 0x0 (0x48, 0x80000000)
...
```
and code in src/decode.c:
```
...
error |= decode_preR13_entities (blocks_start, blocks_end,
                                 blocks_start - blocks_offset, dat, dwg);
...
```
This doesn't work because blocks_end is 0. We need to use block_start + modified block_offset (& 0x00ffffff)

Issue with block table parsing in preR13
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : Issue with block table parsing in preR13
Issue date:

--- start body ---
There is issue in block table parsing in preR13
Example: 
[BLOCK1.DXF.gz](https://github.com/LibreDWG/libredwg/files/8549851/BLOCK1.DXF.gz)
[BLOCK1.DWG.gz](https://github.com/LibreDWG/libredwg/files/8549852/BLOCK1.DWG.gz)

Actual parsing:
```
...
blocks   0x46c (0xfffffb94) - 0x0 (0x48, 0x80000000)
...
```
and code in src/decode.c:
```
...
error |= decode_preR13_entities (blocks_start, blocks_end,
                                 blocks_start - blocks_offset, dat, dwg);
...
```
This doesn't work because blocks_end is 0. We need to use block_start + modified block_offset (& 0x00ffffff)

--- end body ---
