avcodec/mpegvideo: check that the context is initialized in ff_mpv_common_frame_size_change()

The function otherwise would initialize the context without setting context_initialized
alternatively we could set context_initialized

Fixes valgrind anomalies related to ticket 3928
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -1456,6 +1456,9 @@ int ff_mpv_common_frame_size_change(MpegEncContext *s)
 {
     int i, err = 0;
 
+    if (!s->context_initialized)
+        return AVERROR(EINVAL);
+
     if (s->slice_context_count > 1) {
         for (i = 0; i < s->slice_context_count; i++) {
             free_duplicate_context(s->thread_context[i]);