Don't pass NULL to fprintf, if a bogus connection string is given to pg_dump.

Back-patch to all supported branches.

src/bin/pg_dump/pg_backup_db.c

@@ -255,7 +255,8 @@ ConnectDatabase(Archive *AHX,
 	/* check to see that the backend connection was successfully made */
 	if (PQstatus(AH->connection) == CONNECTION_BAD)
 		die_horribly(AH, modulename, "connection to database \"%s\" failed: %s",
-					 PQdb(AH->connection), PQerrorMessage(AH->connection));
+					 PQdb(AH->connection) ? PQdb(AH->connection) : "",
+					 PQerrorMessage(AH->connection));
 
 	/* check for version mismatch */
 	_check_database_version(AH);