Query: Check if $wp_query is set in query loop functions.

This avoids a PHP fatal error if any of these functions are called too early:

* `have_posts()`
* `in_the_loop()`
* `rewind_posts()`
* `the_post()`
* `have_comments()`
* `the_comment()`

bringing some consistency with conditional tags: `is_single()`, `is_home()`, etc.

This commit also removes unnecessary `return` from `the_comment()`, for consistency with `the_post()`. As `WP_Query::the_comment()` does not have a return value, this statement did not have any effect in practice.

Follow-up to [4934], [8807], [16947], [17068], [17083], [49147], [53395], [53396], [53400].

Props vdankbaar, thijso, teunvgisteren, timkersten655, SergeyBiryukov.
Fixes #55722.

git-svn-id: https://develop.svn.wordpress.org/trunk@53429 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/query.php

@@ -939,6 +939,11 @@ function is_main_query() {
  */
 function have_posts() {
 	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		return false;
+	}
+
 	return $wp_query->have_posts();
 }
 
@@ -957,6 +962,11 @@ function have_posts() {
  */
 function in_the_loop() {
 	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		return false;
+	}
+
 	return $wp_query->in_the_loop;
 }
 
@@ -969,6 +979,11 @@ function in_the_loop() {
  */
 function rewind_posts() {
 	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		return;
+	}
+
 	$wp_query->rewind_posts();
 }
 
@@ -981,6 +996,11 @@ function rewind_posts() {
  */
 function the_post() {
 	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		return;
+	}
+
 	$wp_query->the_post();
 }
 
@@ -999,6 +1019,11 @@ function the_post() {
  */
 function have_comments() {
 	global $wp_query;
+
+	if ( ! isset( $wp_query ) ) {
+		return false;
+	}
+
 	return $wp_query->have_comments();
 }
 
@@ -1008,12 +1033,15 @@ function have_comments() {
  * @since 2.2.0
  *
  * @global WP_Query $wp_query WordPress Query object.
- *
- * @return null
  */
 function the_comment() {
 	global $wp_query;
-	return $wp_query->the_comment();
+
+	if ( ! isset( $wp_query ) ) {
+		return;
+	}
+
+	$wp_query->the_comment();
 }
 
 /**