Remove overly strict assertions in transports.c.

This patch removes an overly strict tor_assert() and an ignorable BUG()
expression. Both of these would trigger if a PT was unable to configure
itself during startup. The easy way to trigger this is to configure an
obfs4 bridge where you make the obfs4 process try to bind on a port
number under 1024.

See: https://bugs.torproject.org/31091