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

libmemunreachable/tests/MemUnreachable_test.cpp

@@ -40,7 +40,8 @@ class HiddenPointer {
   volatile uintptr_t ptr_;
 };
 
-static void Ref(void* ptr) {
+// Trick the compiler into thinking a value on the stack is still referenced.
+static void Ref(void** ptr) {
   write(0, ptr, 0);
 }
 
@@ -58,14 +59,14 @@ TEST(MemunreachableTest, stack) {
 
   {
     void* ptr = hidden_ptr.Get();
-    Ref(ptr);
+    Ref(&ptr);
 
     UnreachableMemoryInfo info;
 
     ASSERT_TRUE(GetUnreachableMemory(info));
     ASSERT_EQ(0U, info.leaks.size());
 
-    Ref(ptr);
+    ptr = nullptr;
   }
 
   {