mpegvideo_enc: Check AVCodecContext allocation

libavcodec/mpegvideo_enc.c

@@ -1136,6 +1136,8 @@ static int estimate_best_b_count(MpegEncContext *s)
     int64_t best_rd  = INT64_MAX;
     int best_b_count = -1;
 
+    if (!c)
+        return AVERROR(ENOMEM);
     assert(scale >= 0 && scale <= 3);
 
     //emms_c();