BUG/MINOR: ssl: OCSP callback only registered for first SSL_CTX

BUG/MINOR: ssl: OCSP callback only registered for first SSL_CTX

If multiple SSL_CTXs use the same certificate that has an OCSP response
file on the filesystem, only the first one will have the OCSP callback
set. This bug was introduced by "cc346678d MEDIUM: ssl: Add ocsp_certid
in ckch structure and discard ocsp buffer early" which cleared the
ocsp_response from the ckch_data after it was inserted in the tree,
which prevented subsequent contexts from having the callback registered.

This patch should be backported to 2.8.