If mutt_sasl_client_new returns an error, the callers would ignore the
allocated saslconn resource from sasl_client_new. Be sure to release
these with sasl_dispose as documented in sasl.h. Likewise, let callers
(POP/IMAP) dispose the resource on their error paths. SMTP was already
taken care of. Found with LeakSanitizer in IMAP.