Fix stack test on arm32

The stack test puts a pointer to an allocation on the stack, checks
that there are no leaks, then lets it go out of scope and checks
that libmemunreachable can find a leak.  This works on arm64, but
on arm32 the pointer on the stack doesn't get overwitten and the
leak is not detected.  Rewrite the pointer to be NULL instead.

Test: memunreachable_test
Change-Id: I5959a34cbb572a5d8670270077a85d247a3a4880