src/ngx_http_lua_ssl_certby.c: fix possible null pointer dereference found by coverity

[chipitsine]

   deref_ptr: Directly dereferencing pointer c.
419    if (c->addr_text.len) {
420        p = ngx_snprintf(buf, len, ", client: %V", &c->addr_text);
421        len -= p - buf;
422        buf = p;
423    }
424
   CID 149837 (#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.
425    if (c && c->listening && c->listening->addr_text.len) {
426        p = ngx_snprintf(buf, len, ", server: %V", &c->listening->addr_text);
427        /* len -= p - buf; */
428        buf = p;
429    }

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.