avif: If we only have single image, return false at jumpToNextImage

We were errorneously returning true here, as we do not have any more
images to jump to. If we only have one image, return false.

This avoids the avif handler getting stuck in a loop with only single images.

BUG: 521200
FIXED-IN: 6.28

src/imageformats/avif.cpp

@@ -1181,7 +1181,7 @@ bool QAVIFHandler::jumpToNextImage()
     if (m_decoder->imageIndex >= 0) {
         if (m_decoder->imageCount < 2) {
             m_parseState = ParseAvifSuccess;
-            return true;
+            return false;
         }
 
         if (m_decoder->imageIndex >= m_decoder->imageCount - 1) { // start from beginning