Don't crash if there are no trusted certs

The X509_STORE_CTX_init() docs explicitly allow a NULL parameter for the
X509_STORE. Therefore we shouldn't crash if we subsequently call
X509_verify_cert() and no X509_STORE has been set.

Fixes #2462

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6001)