Fix assertion failure in SSL_set_bio()

If SSL_set_bio() is called with a NULL wbio after a failed connection then
this can trigger an assertion failure. This should be valid behaviour and
the assertion is in fact invalid and can simply be removed.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5966)

(cherry picked from commit bd7775e1)