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
(cherry picked from commit 668e16a0dd1ff56d4beeff5c658d8a2a08dbfac8)

Conflicts:

	libavcodec/h264.c
h264: set ref_count to 0 for intra slices.

CC:libav-stable@libav.org
(cherry picked from commit 437211ae73ef1ed8285b4fed7620502ea4999e11)

Fixes deadlocks waiting for non-existing references with some fuzzed files.

Signed-off-by: Reinhard Tartler <siretart@tauware.de>