avcodec/mpegvideo: update disabled assert() to av_assert0()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -1761,7 +1761,7 @@ void ff_MPV_frame_end(MpegEncContext *s)
             break;
         }
     }
-    assert(i < MAX_PICTURE_COUNT);
+    av_assert0(i < MAX_PICTURE_COUNT);
 #endif
 
     // clear copies, to avoid confusion