func_presencestate: Fix invalid memory access.

When parsing information from AstDB while loading,
it is possible that certain pointers are never
set, which leads to invalid memory access and
then, fatally, invalid free attempts on this memory.
We now initialize to NULL to prevent this.

ASTERISK-30311 #close

Change-Id: I6120681d04fd2c12a9473f35ce95a1f8e74e3929