parsers: initialize MpegEncContext.slice_context_count to 1

The mpeg4 video, H264 and VC-1 parser hold (directly or indirectly)
a MpegEncContext in their private context. Since they do not call the
common mpegvideo init function slice_context_count has explicitly set
to 1.
Prevents a null pointer dereference in the h264 parser and fixes
bug 193.

libavcodec/mpeg4video_parser.c

@@ -99,6 +99,7 @@ static av_cold int mpeg4video_parse_init(AVCodecParserContext *s)
     if (!pc->enc)
         return -1;
     pc->first_picture = 1;
+    pc->enc->slice_context_count = 1;
     return 0;
 }