MINOR: ssl: Keep the actual key length in the certificate_ocsp structure

MINOR: ssl: Keep the actual key length in the certificate_ocsp structure

The OCSP tree entry key is a serialized version of the OCSP_CERTID of
the entry which is stored in a buffer that can be at most 128 bytes.
Depending on the length of the serial number, the actual non-zero part
of the key can be smaller than 128 bytes and this new structure member
allows to know how many of the bytes are filled. It will be useful when
dumping the key (in a "show ssl cert <cert>" output for instance).