Fix g_utf8_validate() out argument transfer mode

The "end" argument is unusual in g_utf8_validate(): it's not a classic out
argument which gets allocated by the called function, but merely points into
one of its input arguments. Thus it is "transfer none".

https://bugzilla.gnome.org/show_bug.cgi?id=672889

glib/gutf8.c

@@ -1597,7 +1597,7 @@ fast_validate_len (const char *str,
  * g_utf8_validate:
  * @str: (array length=max_len) (element-type guint8): a pointer to character data
  * @max_len: max bytes to validate, or -1 to go until NUL
- * @end: (allow-none) (out): return location for end of valid data
+ * @end: (allow-none) (out) (transfer none): return location for end of valid data
  * 
  * Validates UTF-8 encoded text. @str is the text to validate;
  * if @str is nul-terminated, then @max_len can be -1, otherwise