avcodec/h264dec: Fix regression with "make fate-h264-attachment-631 THREADS=8"

This treats the case of no slices like no frames which it basically is.

The field is added to the context as other nal related fields are also there
and passing the has_slices field per *arguments is ugly and not consistent

Found-by: ubitux
Approved-by: ubitux

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