BUILD: ssl: avoid a potential null-dereference warning on OpenSSL 1.0.2

BUILD: ssl: avoid a potential null-dereference warning on OpenSSL 1.0.2

On OpenSSL 1.0.2, X509_CRL_get_issuer() is a raw macro expanding to
(x)->crl->issuer instead of an opaque accessor function, so gcc's
inliner sees straight through to the dereference in
ssl_ctx_hash_store(). It cannot prove that the X509_CRL returned by
X509_OBJECT_get0_X509_CRL() is non-NULL, and flags it under
-Wnull-dereference, which "make ERR=1" turns into a hard error:

    x509.h:605:45: error: potential null pointer dereference [-Werror=null-dereference]

Add an explicit NULL check on crl before using it, the same way
ssl_ctx_hash_X509() already guards against a NULL certificate a few
lines above. This only affects builds against OpenSSL 1.0.2, where the
macro form is used; newer OpenSSL versions hide the access behind a
real function and never triggered the warning.

This must be backported in every stable branches along with the sid_ctx
fixes.