Language Pack Upgrader: Check return value before accessing the first array key.

Prevents a fatal error when Language_Pack_Upgrader::bulk_upgrade() returns a WP_Error object.

fixes #29470.

git-svn-id: https://develop.svn.wordpress.org/trunk@29675 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-upgrader.php

@@ -1223,9 +1223,16 @@ class Language_Pack_Upgrader extends WP_Upgrader {
 	}
 
 	public function upgrade( $update = false, $args = array() ) {
-		if ( $update )
+		if ( $update ) {
 			$update = array( $update );
+		}
+
 		$results = $this->bulk_upgrade( $update, $args );
+
+		if ( ! is_array( $results ) ) {
+			return $results;
+		}
+
 		return $results[0];
 	}