The purpose of the compress() wrapper my_compress_buffer() was twofold:
silence Valgrind warnings about uninitialized memory access before
zlib 1.2.4, and have PERFORMANCE_SCHEMA instrumentation of some zlib
related memory allocation. Because of PERFORMANCE_SCHEMA, we cannot
trivially replace my_compress_buffer() with compress().

az_open(): Remove a crc32() call. Any CRC of the empty string is 0.