OPENSSL_hexstr2buf_ex(): Handle zero-length input correctly

OPENSSL_hexstr2buf_ex(): Handle zero-length input correctly

In case of zero-length input the code wrote one byte
before the start of the output buffer. The length
of the output was also reported incorrectly in this case.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24770)