Prevent the pixmap cache from crashing if it's been destroyed

In case the static destructor has already been run, make sure we don't
crash. This shouldn't happen, but could happen if the QApplication
destructor is run in a weird order (after the static destructors have
begun running).

That's not usually a case we'd fix (unsupported), but since this change
improves the code and also avoids creating the pixmap cache if it hadn't
been used up until this point, the change is a net benefit.

Task-number: QTBUG-48709
Change-Id: Ia505aece07bf4e13a1faffff140f3e119cfc773e
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/gui/image/qpixmapcache.cpp

@@ -642,7 +642,8 @@ void QPixmapCache::remove(const Key &key)
 void QPixmapCache::clear()
 {
     QT_TRY {
-        pm_cache()->clear();
+        if (pm_cache.exists())
+            pm_cache->clear();
     } QT_CATCH(const std::bad_alloc &) {
         // if we ran out of memory during pm_cache(), it's no leak,
         // so just ignore it.