nat: Mention when we do not find a BSC for a given token.

This might help to identify what is wrong with the config
of the BSC. Also using the result of TLVP_VAL as a char
pointer looks suspicious...
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 697b162..9a5da0b 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -481,9 +481,11 @@
 			bsc->cfg = conf;
 			bsc_del_timer(&bsc->id_timeout);
 			LOGP(DNAT, LOGL_NOTICE, "Authenticated bsc nr: %d lac: %d\n", conf->nr, conf->lac);
-			break;
+			return;
 		}
 	}
+
+	LOGP(DNAT, LOGL_ERROR, "No bsc found for token %s.\n", token);
 }
 
 static int forward_sccp_to_msc(struct bsc_connection *bsc, struct msgb *msg)