Use NULL instead of 0 for null pointer constant

There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/core/ngx_thread_pool.c

@@ -207,7 +207,7 @@ ngx_thread_pool_exit_handler(void *data, ngx_log_t *log)
 
     *lock = 0;
 
-    pthread_exit(0);
+    pthread_exit(NULL);
 }
 
 

src/event/ngx_event_openssl_cache.c

@@ -694,7 +694,7 @@ ngx_ssl_cache_pkey_create(ngx_ssl_cache_key_t *id, char **err, void *data)
             return NULL;
         }
 
-        pkey = ENGINE_load_private_key(engine, (char *) last, 0, 0);
+        pkey = ENGINE_load_private_key(engine, (char *) last, NULL, NULL);
 
         if (pkey == NULL) {
             *err = "ENGINE_load_private_key() failed";

src/event/quic/ngx_event_quic.c

@@ -964,7 +964,7 @@ ngx_quic_handle_payload(ngx_connection_t *c, ngx_quic_header_t *pkt)
     qc = ngx_quic_get_connection(c);
 
     qc->error = 0;
-    qc->error_reason = 0;
+    qc->error_reason = NULL;
 
     c->log->action = "decrypting packet";
 

src/http/modules/ngx_http_gunzip_filter_module.c

@@ -304,7 +304,7 @@ ngx_http_gunzip_filter_inflate_start(ngx_http_request_t *r,
 {
     int  rc;
 
-    ctx->zstream.next_in = Z_NULL;
+    ctx->zstream.next_in = NULL;
     ctx->zstream.avail_in = 0;
 
     ctx->zstream.zalloc = ngx_http_gunzip_filter_alloc;

src/http/modules/ngx_http_ssi_filter_module.c

@@ -820,7 +820,7 @@ ngx_http_ssi_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
                 }
 
                 for (prm = cmd->params; prm->name.len; prm++) {
-                    if (prm->mandatory && params[prm->index] == 0) {
+                    if (prm->mandatory && params[prm->index] == NULL) {
                         ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                                       "mandatory \"%V\" parameter is absent "
                                       "in \"%V\" SSI command",

src/os/unix/ngx_time.c

@@ -43,7 +43,7 @@ ngx_timezone_update(void)
     struct tm  *t;
     char        buf[4];
 
-    s = time(0);
+    s = time(NULL);
 
     t = localtime(&s);