TLSRecv now '\0'-terminates all received data.

This has been the case for the non-TLS counterpart (RecvSocketStream())
so TLSRecv() must follow the same logic to make sure all (hundreds)
callers of ReceiveTransaction() are safe.
Clarify comment.
Merge pull request #1641 from jimis/reverse_dns_access_rules

TLSRecv now '\0'-terminates all received data.