LWIS: add lock for emitted_event_list in subscribe_work_func

All accesses to emitted_event_list have been added lock except
list_empty in subscribe_work_func. Add lock for it.
Also add check for trigger_event allocate result to prevent
null pointer access.

Bug: 174808156
Test: camera CTS
Signed-off-by: Holmes Chou <holmeschou@google.com>
Change-Id: I73d4073c51d0a5948b5c3bff5c4b8b776d3ebba7