Treat QTestFontEngine properly

Do not try to fill fontDef when it's a test font engine, otherwise
it will cause a crash because desc.family is 0.

Change-Id: I008b784bae3e60a25c33c285d944442a3616a9a2
Reviewed-on: http://codereview.qt-project.org/5103
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>

src/gui/text/qfontdatabase_qpa.cpp

@@ -260,7 +260,7 @@ QFontDatabase::findFont(int script, const QFontPrivate *fp,
         FM_DEBUG("  NO MATCH FOUND\n");
     }
 
-    if (engine) {
+    if (engine && engine->type() != QFontEngine::TestFontEngine) {
         initFontDef(desc, request, &engine->fontDef);
 
         if (fp) {