mpegvideo: clear overread in clear_context

Otherwise the h263p decoder can try to copy overread bytes, even though
buffer is NULL.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mpegvideo.c

@@ -824,6 +824,7 @@ static void clear_context(MpegEncContext *s)
 
     s->parse_context.buffer = NULL;
     s->parse_context.buffer_size = 0;
+    s->parse_context.overread = 0;
     s->bitstream_buffer = NULL;
     s->allocated_bitstream_buffer_size = 0;
     s->picture          = NULL;