convert relayd to use new ibuf function

[Claudio Jeker]

Instead of ibuf_reserve() just use ibuf_add_zero(buf, 1) to add a
NUL byte to the buffer.

There is more needed in here but lets start small.
-- 
:wq Claudio

Index: check_tcp.c
===================================================================
RCS file: /cvs/src/usr.sbin/relayd/check_tcp.c,v
retrieving revision 1.58
diff -u -p -r1.58 check_tcp.c
--- check_tcp.c 18 Sep 2021 19:44:46 -0000      1.58
+++ check_tcp.c 10 Jun 2023 07:36:17 -0000
@@ -344,17 +344,14 @@ check_http_code(struct ctl_tcp_event *ct
        char            *head;
        char             scode[4];
        const char      *estr;
-       u_char          *b;
        int              code;
        struct host     *host;
 
        /*
         * ensure string is nul-terminated.
         */
-       b = ibuf_reserve(cte->buf, 1);
-       if (b == NULL)
+       if (ibuf_add_zero(cte->buf, 1) == -1)
                fatal("out of memory");
-       *b = '\0';
 
        head = cte->buf->buf;
        host = cte->host;
@@ -398,17 +395,14 @@ int
 check_http_digest(struct ctl_tcp_event *cte)
 {
        char            *head;
-       u_char          *b;
        char             digest[SHA1_DIGEST_STRING_LENGTH];
        struct host     *host;
 
        /*
         * ensure string is nul-terminated.
         */
-       b = ibuf_reserve(cte->buf, 1);
-       if (b == NULL)
+       if (ibuf_add_zero(cte->buf, 1) == -1)
                fatal("out of memory");
-       *b = '\0';
 
        head = cte->buf->buf;
        host = cte->host;