Added a pertinent assertion.
Assert known constraints, log debug on any success.

ServerTLSPeek() would log the data it peeked, but only when deciding
it has TLS; I'd like to be able to see it also for classic.

As recv() definitely shouldn't return more than the buffer size it was
given, assert this, tweak the first check (any negative is failure,
not just -1, albeit only -1 should happen) and simplify the rest.  The
TLS case wasn't checking got == peek_size (as the CAUTH case did), but
can take this for granted (given the assertions) now.