Handle negative return values from syncache_expand().

These errors do not clear so to NULL, so the existing check was
treating these failures as success. The rest of do_pass_establish()
then tried to use the listen socket as if it was a connection socket
newly created by syncache_expand().

In addition, for negative return values, do not send a RST to the
peer.

Reported by: Sony Arpita Das @ Chelsio
Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D28243