Prevent crashing when FcFontList fails

FcFontList can return null pointer in failure cases which would lead to
null pointer dereference further down.

Pick-to: 5.15 6.2 6.3
Change-Id: I6b407cf2f27ead9eb471d3ee7a521468cebf7572
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/text/unix/qfontconfigdatabase.cpp

@@ -593,6 +593,8 @@ void QFontconfigDatabase::populateFontDatabase()
         fonts = FcFontList(nullptr, pattern, os);
         FcObjectSetDestroy(os);
         FcPatternDestroy(pattern);
+        if (!fonts)
+            return;
     }
 
     for (int i = 0; i < fonts->nfont; i++)