Fixed g_mutex_clear() to fully undo g_mutex_init()

In order to fully undo the effects of g_mutex_init(),
it is necessary to reset the internal mutex pointer
back to NULL so that a later call to g_mutex_init()
actually works as expected.

glib/gthread-posix.c

@@ -184,12 +184,13 @@ g_mutex_init (GMutex *mutex)
  * Calling g_mutex_clear() on a locked mutex leads to undefined
  * behaviour.
  *
- * Sine: 2.32
+ * Since: 2.32
  */
 void
 g_mutex_clear (GMutex *mutex)
 {
   g_mutex_impl_free (mutex->p);
+  mutex->p = NULL;
 }
 
 /**