avcodec/h264: remove list_count and ref_count clearing

The code conflicts with moving the h264_init_ps() call point

Without this, ff_h264_parse_ref_count() fills ref and list count and
h264_init_ps() subsequently wipes them out on a "success" path.
Subsequently things crash as the wiped fields are used.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
