TCP: Fix bug: update cached PMTU when PMTU changes in SYN_SENT.
TCP: Fix bug: update cached PMTU when PMTU changes in SYN_SENT.

Also remove redundant "con != nullptr" check.

ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter