Add QPixmapIconEngine::pixmap pointer check

Dereference after null check (FORWARD_NULL)5.
var_deref_op: Dereferencing null pointer pe.

pe pointer it's being checked at the beginning of the function so it
implies pe might be null.

Coverity-Id: 11106
Change-Id: Ie88b27877a46cdd20a317fb5e21c3fdec1b99dda
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/gui/image/qicon.cpp

@@ -294,7 +294,7 @@ QPixmap QPixmapIconEngine::pixmap(const QSize &size, QIcon::Mode mode, QIcon::St
 
     QString key = QLatin1String("qt_")
                   % HexString<quint64>(pm.cacheKey())
-                  % HexString<uint>(pe->mode)
+                  % HexString<uint>(pe ? pe->mode : QIcon::Normal)
                   % HexString<quint64>(QGuiApplication::palette().cacheKey())
                   % HexString<uint>(actualSize.width())
                   % HexString<uint>(actualSize.height());