avcodec/pngdec: Clean up on av_frame_ref() failure

Fixes: memleak
Fixes: 3203/clusterfuzz-testcase-minimized-4514553595428864

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5480e82d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngdec.c

@@ -888,7 +888,7 @@ exit_loop:
     metadata   = NULL;
 
     if ((ret = av_frame_ref(data, s->picture.f)) < 0)
-        return ret;
+        goto the_end;
 
     *got_frame = 1;