cert-cred: fix possible segfault when resetting cert retrieval function

cert-cred: fix possible segfault when resetting cert retrieval function

Reset get_cert_callback3 callback to NULL if provided callback is NULL.
Otherwise after the certificate request call_legacy_cert_cb1 /
call_legacy_cert_cb2 will try to unconditionally call legacy_cert_cb1 /
legacy_cert_cb2 callback (set to NULL) leading to segfault.

Fixes: 9829ef9a3ca06d60472599df7c74ebb9a53f1fe2
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>