gstring: Mark the return value from g_string_free() as nullable

It’s NULL iff free_segment is TRUE, so the annotation doesn’t quite
capture all the function definition, but is a safe over-estimate of the
return value’s nullability.

https://bugzilla.gnome.org/show_bug.cgi?id=719966

glib/gstring.c

@@ -203,7 +203,7 @@ g_string_new_len (const gchar *init,
  * it's %FALSE, the caller gains ownership of the buffer and must
  * free it after use with g_free().
  *
- * Returns: the character data of @string
+ * Returns: (nullable): the character data of @string
  *          (i.e. %NULL if @free_segment is %TRUE)
  */
 gchar *