tcp rack: fix memory corruption

When in rack_output() jumping to the label out, don't write errno into
the log buffer, since the pointer is not initialized.

Reported by:		Coverity Scan
CID:			1523773
Reviewed by:		rscheff
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D44647

sys/netinet/tcp_stacks/rack.c

@@ -23718,12 +23718,12 @@ send:
 			mtu = inp->inp_route.ro_nh->nh_mtu;
 	}
 #endif				/* INET */
-
-out:
 	if (lgb) {
 		lgb->tlb_errno = error;
 		lgb = NULL;
 	}
+
+out:
 	/*
 	 * In transmit state, time the transmission and arrange for the
 	 * retransmit.  In persist state, just set snd_max.