avcodec/h264_slice: Limit max_contexts when slice_context_count is initialized

      Fixes out of array access
Fixes: 1430e9c43fae47a24c179c7c54f94918/signal_sigsegv_421427_2049_f2192b6829ab6e0eefcb035329c03c60.264

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>