Stocks Fall on Security, Earnings Worries  NEW YORK (Reuters) - U.S. stocks fell in extremely thin  volume on Monday as security concerns surrounding the  Republican convention in New York and worries about an update  from technology bellwether Intel Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=INTC.O target=/stocks/quickinfo/fullquote"&gt;INTC.O&lt;/A&gt; this week kept  investors on the sidelines.