Add an explicit check for a malformed AS segment with (segment length 0),

Add an explicit check for a malformed AS segment with (segment length 0),
avoiding division by zero when deciding whether it contains 2- or 4-byte ASNs.
Refactor TCHECK calls to ensure proper coverage.

From Kevin Reay, ok canacar with wording tweak (I used "malformed" rather
than canacar's suggested "invalid size" or Kevin's original "empty").