darwin: fix thinko (free thread port after threads are discovered).

Due to a thinko, a message could be not understood and ignored.  The result
was a dead-lock (gdb is waiting for an event that never happen).  The port
of the thread was deallocated before new threads are discovered.  As a
consequence, the origin of the message was unknown (instead of being
linked to the newly created thread).

gdb/
	* darwin-nat.c (darwin_check_new_threads): Fix port leak, add
	comments.
	(darwin_decode_exception_message): Free port only after use.