Windows: Fix exit crash of GUI applications when deleting argv[].

When passing Qt arguments followed by normal arguments,
a double deletion may occur due to Qt shifting argv.
For example:

argv[] = app -qwindowgeometry +50+50 some_arg <null>

becomes:

argv[] = app some_arg <null> some_arg <null>

Terminate deletion when encountering the null pointer.

Change-Id: I5279955b6bd463f5858d6e5e8e16a1f5d0945652
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/winmain/qtmain_win.cpp

@@ -111,7 +111,7 @@ extern "C" int APIENTRY WinMain(HINSTANCE, HINSTANCE, LPSTR /*cmdParamarg*/, int
     argv[argc] = Q_NULLPTR;
     LocalFree(argvW);
     const int exitCode = main(argc, argv);
-    for (int i = 0; i < argc; ++i)
+    for (int i = 0; i < argc && argv[i]; ++i)
         delete [] argv[i];
     delete [] argv;
     return exitCode;