Fannie Mae Slides on Accounting Issues  NEW YORK (Reuters) - Fannie Mae &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=FNM.N target=/stocks/quickinfo/fullquote"&gt;FNM.N&lt;/A&gt; shares fell as much  as 3.5 percent on Friday, bringing losses for the week to about  16 percent, as investors grew wary of potential widespread  accounting problems at the No. 1 U.S. housing finance company.