avcodec/h264_refs: Change default case to av_assert0() to suppress warning

Should fix "libavcodec/h264_refs.c:372:13: warning: variable 'i' is used uninitialized whenever switch default is taken"

Found-by: durandal_17
Suggested-by: jkqxz
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
