Avoid PHP notices when checking termmeta capabilities against a non-existent term.

Previously, checks like `current_user_can( 'edit_term_meta', $term_id )`
returned the proper value, but generated a PHP notice due to the fact
that `get_term( $term_id )` could, in certain instances, return
`WP_Error` objects.

Props caercam.
Fixes #40891.
Built from https://develop.svn.wordpress.org/trunk@40999


git-svn-id: http://core.svn.wordpress.org/trunk@40849 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/capabilities.php

@@ -280,7 +280,7 @@ function map_meta_cap( $cap, $user_id ) {
 
 			case 'term':
 				$term = get_term( $object_id );
-				if ( ! $term ) {
+				if ( ! $term instanceof WP_Term ) {
 					break;
 				}