Fix segfault in stdio with non-SMTP MIME ACL.

When injecting a message locally in non-SMTP mode, and with MIME ACLs
configured, if the ACL rejected the message, Exim would try to
`fprintf(NULL, "%s", the_message)`.  This fixes that.

Most ACLs are plumbed in SMTP-only and looking through the others in
receive.c, they all appear to be safely guarded, so it was just this one
that slipped through.

Crash report and assistance tracking down the root cause from Warren
Baker.