apparently calling match on a string with an index past the end is now fatal

assert on loading ooo55345-1.ppt etc

Change-Id: Ie76720e511674ddd883f7e397442a84838c1a1f4

sc/source/filter/excel/xihelper.cxx

@@ -623,7 +623,7 @@ void XclImpUrlHelper::DecodeUrl(
 
     sal_Unicode cCurrDrive = 0;
     OUString aDosBase( INetURLObject( rRoot.GetBasePath() ).getFSysPath( INetURLObject::FSYS_DOS ) );
-    if( aDosBase.match(":\\", 1) )
+    if (!aDosBase.isEmpty() && aDosBase.match(":\\", 1))
         cCurrDrive = aDosBase[0];
 
     const sal_Unicode* pChar = rEncodedUrl.getStr();