If the len2 argument to crc32_combine() is zero, then the crc2
argument must also be zero.
fast_crc32_combine() explicitly checks for len2==0, in which case it
ignores crc2 (which is the same as if it were zero).
zlib's crc32_combine() used to have that check prior to version
1.2.12, but then lost it, making its necessary for callers to be more
careful.
Also add the len2==0 check to the dummy fast_crc32_combine()
implementation, because it delegates to zlib's.
Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>
Closes#10731
1e7360ef6d