I18N: Add hardening for invalid meta lines in MO files.

Props swissspidy, pander.
Fixes #61794.

git-svn-id: https://develop.svn.wordpress.org/trunk@58922 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/l10n/class-wp-translation-file-mo.php

@@ -154,7 +154,7 @@ protected function parse_file(): bool {
 			// Metadata about the MO file is stored in the first translation entry.
 			if ( '' === $original ) {
 				foreach ( explode( "\n", $translation ) as $meta_line ) {
-					if ( '' === $meta_line ) {
+					if ( '' === $meta_line || ! str_contains( $meta_line, ':' ) ) {
 						continue;
 					}