h264: do not copy ref count/ref2frm when updating per-frame context

They are filled in decode_slice_header() anyway.
h264: on reference overflow, reset the reference count to 0, not 1.

Since decode_slice_header() returns before the reference lists are
constructed, there are zero valid references.

CC:libav-stable@libav.org
h264: set ref_count to 0 for intra slices.

CC:libav-stable@libav.org