When querying for terms, do not assume that $taxonomies is a 0-indexed array.

In [25108], the logic of term queries was modified to avoid assuming
that the `taxonomies` array was numerically indexed. See #23506. This
fix was inadvertantly reverted during the refactor in [25162].

Props david.binda.
Fixes #41113.

git-svn-id: https://develop.svn.wordpress.org/trunk@40924 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-term-query.php

@@ -429,7 +429,7 @@ class WP_Term_Query {
 			foreach ( $exclude_tree as $extrunk ) {
 				$excluded_children = array_merge(
 					$excluded_children,
-					(array) get_terms( $taxonomies[0], array(
+					(array) get_terms( reset( $taxonomies ), array(
 						'child_of' => intval( $extrunk ),
 						'fields' => 'ids',
 						'hide_empty' => 0