fix a crash with empty string

If it's named nLast, then it's supposed to point to the last character,
which, with an empty string, logically means it should point before
the start of the string.

Change-Id: I248fbe80c1c4b1c2c8d39f0bcf772f9b85415e4a

xmloff/source/style/fonthdl.cxx

@@ -85,12 +85,10 @@ sal_Bool XMLFontFamilyNamePropHdl::importXML( const OUString& rStrImpValue, uno:
     {
         sal_Int32 nFirst = nPos;
         nPos = ::sax::Converter::indexOfComma( rStrImpValue, nPos );
-        sal_Int32 nLast = (-1 == nPos ? rStrImpValue.getLength() : nPos);
-        if( nLast > 0 )
-            nLast--;
+        sal_Int32 nLast = (-1 == nPos ? rStrImpValue.getLength() - 1 : nPos - 1);
 
         // skip trailing blanks
-        while( sal_Unicode(' ') == rStrImpValue[nLast] && nLast > nFirst )
+        while( nLast > nFirst && sal_Unicode(' ') == rStrImpValue[nLast] )
             nLast--;
 
         // skip leading blanks