ipa: Be consistent in checking conn->cfg (Coverity)

Coverity complains about checking connection->cfg in
bsc_close_connection() at one place but not at the second.

This patch fixes this by adding a check before accessing cfg when
generating the 'partial message' log message.

Fixes: Coverity CID 1195180
Sponsored-by: On-Waves ehf

openbsc/src/osmo-bsc_nat/bsc_nat.c

@@ -916,7 +916,7 @@ void bsc_close_connection(struct bsc_connection *connection)
 
 	if (connection->pending_msg) {
 		LOGP(DNAT, LOGL_ERROR, "Dropping partial message on connection %d.\n",
-		     connection->cfg->nr);
+		     connection->cfg ? connection->cfg->nr : -1);
 		msgb_free(connection->pending_msg);
 		connection->pending_msg = NULL;
 	}