bta_jv_act: Avoid writing through NULL/freed pointer

Before writing to 't->init_called', we know that 't' might be
NULL (there's an explicit check), so we add another check here
to assure it's not NULL.  Furthermore, we're possibly freeing
't' prior to writing to 't->init_called'.  So we set 't' NULL
in that case, so our new NULL check will let us avoid corrupting
memory we no longer own.

Test: TreeHugger
Change-Id: Iaa246d5190f5f99610dace24707e74a846df3cf3