Remove a check in channeltls.c that could never fail.

We were checking whether a 8-bit length field had overflowed a
503-byte buffer. Unless somebody has found a way to store "504" in a
single byte, it seems unlikely.

Fix for 10313 and 9980. Based on a pach by Jared L Wong. First found
by David Fifield with STACK.
Merge remote-tracking branch 'public/bug10131_024'
Fix get_configured_bridge_by_addr_port_digest(.,.,NULL)

The old behavior was that NULL matched only bridges without known
identities; the correct behavior is that NULL should match all
bridges (assuming that their addr:port matches).