* mu-util.[ch]: add mu_util_locale_is_utf8, handle invalid utf8 string more gracefully

src/mu-util.h

@@ -135,6 +135,13 @@ int mu_util_create_writeable_fd (const char* path, mode_t mode,
 gboolean mu_util_is_local_file (const char* path);
 
 
+/**
+ * is the current locale utf-8 compatible? 
+ * 
+ * @return TRUE if it's utf8 compatible, FALSE otherwise
+ */
+gboolean mu_util_locale_is_utf8 (void) G_GNUC_CONST;
+
 /**
  * write a string (assumed to be in utf8-format) to a stream,
  * converted to the current locale