Fix -Werror=format-truncation= warnings in blr_slave.c

The errors were caused by the fact that the server name
MAX_SERVER_NAME_LEN is 1024 bytes long whereas the buffer was 251 bytes.

server/modules/routing/binlogrouter/blr_slave.c

@@ -1480,7 +1480,7 @@ static int
 blr_slave_send_slave_status(ROUTER_INSTANCE *router, ROUTER_SLAVE *slave)
 {
     GWBUF *pkt;
-    char column[251] = "";
+    char column[MAX_SERVER_NAME_LEN + 1] = "";
     uint8_t *ptr;
     int len, actual_len, col_len, seqno, ncols, i;
     char *dyn_column = NULL;