winrt: Fix ill-constructed loop

That loop could never have worked properly.
Identified by analyze compile switch.

Change-Id: I5e987dc9f5c57d3ffbcf054b7b417b506c02e7e1
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/winrt/qwinrtfileengine.cpp

@@ -92,7 +92,7 @@ public:
         }
 
         firstDot = fileName.size();
-        for (int i = lastSeparator; i > fileName.size(); ++i) {
+        for (int i = lastSeparator; i < fileName.size(); ++i) {
             if (fileName.at(i).unicode() == '.') {
                 firstDot = i;
                 break;