gst/elements/: Protect last_message with lock.

gst/elements/: Protect last_message with lock.

Original commit message from CVS:
* gst/elements/gstfakesink.c: (gst_fake_sink_get_property),
(gst_fake_sink_event), (gst_fake_sink_preroll),
(gst_fake_sink_render), (gst_fake_sink_change_state):
* gst/elements/gstfakesrc.c: (gst_fake_src_event_handler),
(gst_fake_src_get_property), (gst_fake_src_create),
(gst_fake_src_stop):
* gst/elements/gstidentity.c: (gst_identity_stop):
Protect last_message with lock.