ff_alloc_picture: free tables in case of dimension mismatches

Fixes memory corruption
In theory this should not be needed but its better to check at
one place than to hope 50 other places set the flags correctly

libavcodec/mpegvideo.c

@@ -411,6 +411,11 @@ int ff_alloc_picture(MpegEncContext *s, Picture *pic, int shared)
 {
     int i, ret;
 
+    if (pic->qscale_table_buf)
+        if (pic->mbskip_table_buf->size < s->mb_stride * s->mb_height + 2 ||
+            pic->qscale_table_buf->size < s->mb_stride * (s->mb_height + 1) + 1 + s->mb_stride)
+            free_picture_tables(pic);
+
     if (shared) {
         assert(pic->f.data[0]);
         pic->shared = 1;