plugins/elements/gsttee.*: Protect pad_alloc with a new lock so that we can be sure...

plugins/elements/gsttee.*: Protect pad_alloc with a new lock so that we can be sure that nothing is performing a pad_...

Original commit message from CVS:
Patch by: Ole André Vadla Ravnås  <ole.andre.ravnas at tandberg com>
* plugins/elements/gsttee.c: (gst_tee_finalize), (gst_tee_init),
(gst_tee_request_new_pad), (gst_tee_release_pad),
(gst_tee_find_buffer_alloc), (gst_tee_buffer_alloc):
* plugins/elements/gsttee.h:
Protect pad_alloc with a new lock so that we can be sure that nothing is
performing a pad_alloc when removing the pad. Fixes #547835.
* tests/check/elements/tee.c: (buffer_alloc_harness_setup),
(buffer_alloc_harness_teardown), (app_thread_func),
(final_sinkpad_bufferalloc), (GST_START_TEST), (tee_suite):
Added testcase for shutdown race.