Test av_buffer_alloc, av_buffer_allocz, av_buffer_create with
custom free callback, AV_BUFFER_FLAG_READONLY, av_buffer_ref,
av_buffer_is_writable, av_buffer_get_ref_count,
av_buffer_make_writable, av_buffer_realloc (including from NULL),
av_buffer_replace (including with NULL), av_buffer_pool
init/get/uninit cycle, av_buffer_pool_init2 with custom alloc
and pool_free callbacks, av_buffer_pool_buffer_get_opaque, and
OOM paths via av_max_alloc.
Coverage for libavutil/buffer.c: 0.00% -> 90.19%
Remaining uncovered lines are mutex init failures and
secondary allocation failure paths.
