Cocoa: Don't crash at toolbar cleanup time.

Add missing iterator increment.

Change-Id: Id663c38859b89c29009f67205da0fdd404a455c0
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -487,6 +487,7 @@ void QCocoaIntegration::clearToolbars()
     QHash<QWindow *, NSToolbar *>::const_iterator it = mToolbars.constBegin();
     while (it != mToolbars.constEnd()) {
         [it.value() release];
+        ++it;
     }
     mToolbars.clear();
 }