daemon: guard NULL REMOTE_PORT in execute() logging

daemon: guard NULL REMOTE_PORT in execute() logging

REMOTE_ADDR and REMOTE_PORT are both set by the same code path in
handle(), so when the existing REMOTE_ADDR check passes, REMOTE_PORT
is guaranteed to be non-NULL.  Guard REMOTE_PORT as well so that a
future change that breaks this invariant does not pass NULL to
printf's %s, which is undefined behavior.

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>