chore: fix possible null pointer dereference found by Coverity

[chipitsine]

785    buf = p;
786
   deref_ptr: Directly dereferencing pointer c.
787    if (c->addr_text.len) {
788        p = ngx_snprintf(buf, len, ", client: %V", &c->addr_text);
789        len -= p - buf;
790        buf = p;
791    }
792
   CID 251610 (#1 of 1): Dereference before null check (REVERSE_INULL)check_after_deref: Null-checking c suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
793    if (c && c->listening && c->listening->addr_text.len) {
794        p = ngx_snprintf(buf, len, ", server: %V", &c->listening->addr_text);
795        /* len -= p - buf; */
796        buf = p;
797    }