w: initialize save_p to silence 'may be used uninitilized'

After r351379 save_p may be used uninitialized.  Set it to NULL before
first assignment so that a later NULL check will work correctly.

Reported by:		CI system for gcc platforms
MFC after:		1 week
X-MFC with:		351379 (karels)

usr.bin/w/w.c

@@ -209,6 +209,7 @@ main(int argc, char *argv[])
 	if (*argv)
 		sel_users = argv;
 
+	save_p = NULL;
 	setutxent();
 	for (nusers = 0; (utmp = getutxent()) != NULL;) {
 		struct addrinfo hints, *res;