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.