The memory referenced via it could have been freed already
(by the `update_uac_request()`). Set the original pointer to NULL
and check in the caller's function if it has been assigned to NULL
before actually using.
Fixes:
Read from pointer after free (USE_AFTER_FREE)
41. deref_after_free: Dereferencing freed pointer p_msg.
Change-Id: I07ee9360f996c8723ba46c99f512ef9562ff75c3