avcodec/mpegvideo: Do not clear the parse context during init

It is allocated before, this cannot work
Fixes Ticket5613
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpegvideo.c

@@ -822,9 +822,7 @@ static void clear_context(MpegEncContext *s)
     s->sc.b_scratchpad =
     s->sc.obmc_scratchpad = NULL;
 
-    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;