Assert to ensure ftell() behaves as expected by later code

tinyxml2.cpp

@@ -1954,6 +1954,7 @@ XMLError XMLDocument::LoadFile( FILE* fp )
         SetError( XML_ERROR_FILE_READ_ERROR, 0, 0 );
         return _errorID;
     }
+    TIXMLASSERT( filelength >= 0 );
 
     if ( !LongFitsIntoSizeTMinusOne<>::Fits( filelength ) ) {
         // Cannot handle files which won't fit in buffer together with null terminator