SPARC assembly: Don't file aes-cbc on T4 with small sizes.

SPARC assembly: Don't file aes-cbc on T4 with small sizes.

The "openssl speed -testmode -seconds 1 -bytes 1 aes-128-cbc" test
revealed that the assembly code is crashing if length is less than 16.
The code shifts the provided length by 4 and than subtracts one until
the length hits zero. If it was already zero then it underflows the
counter and continues until it segfaults on reading or writing.

Replace the check against 0 with less than 15.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25637)