Reset error message at PQreset()

If you call PQreset() repeatedly, and the connection cannot be
re-established, the error messages from the failed connection attempts
kept accumulating in the error string.

Fixes bug #11455 reported by Caleb Epstein. Backpatch to all supported
versions.

src/interfaces/libpq/fe-connect.c

@@ -2895,6 +2895,7 @@ closePGconn(PGconn *conn)
 										 * absent */
 	conn->asyncStatus = PGASYNC_IDLE;
 	pqClearAsyncResult(conn);	/* deallocate result */
+	resetPQExpBuffer(&conn->errorMessage);
 	pg_freeaddrinfo_all(conn->addrlist_family, conn->addrlist);
 	conn->addrlist = NULL;
 	conn->addr_cur = NULL;