QMngHandler: Avoid endless loop

In case the animation only has one frame, like Waves1.mng in the bug
report.

Task-number: QTBUG-42321
Change-Id: Id6616f4f51c1b6f06f7e4677cbd9e56135d2a52b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/imageformats/mng/qmnghandler.cpp

@@ -342,7 +342,7 @@ bool QMngHandlerPrivate::jumpToImage(int imageNumber)
 bool QMngHandlerPrivate::jumpToNextImage()
 {
     const int numImages = imageCount();
-    return numImages && jumpToImage((currentImageNumber() + 1) % numImages);
+    return numImages > 1 && jumpToImage((currentImageNumber() + 1) % numImages);
 }
 
 int QMngHandlerPrivate::nextImageDelay() const