avcodec/mpegvideo_enc: Update the buffer size as more slices are merged

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 561d3a57)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo_enc.c

@@ -3742,6 +3742,8 @@ static int encode_picture(MpegEncContext *s, int picture_number)
     }
     s->avctx->execute(s->avctx, encode_thread, &s->thread_context[0], NULL, context_count, sizeof(void*));
     for(i=1; i<context_count; i++){
+        if (s->pb.buf_end == s->thread_context[i]->pb.buf)
+            set_put_bits_buffer_size(&s->pb, FFMIN(s->thread_context[i]->pb.buf_end - s->pb.buf, INT_MAX/8-32));
         merge_context_after_encode(s, s->thread_context[i]);
     }
     emms_c();