Fix GC memory overhead accounting.

There was some missing null checks.

Bug: 16238192

Change-Id: Iaf8d752db5f21e76f668c0066a063239ff374eee
diff --git a/runtime/gc/accounting/gc_allocator.cc b/runtime/gc/accounting/gc_allocator.cc
index 49d84fa..ff6a135 100644
--- a/runtime/gc/accounting/gc_allocator.cc
+++ b/runtime/gc/accounting/gc_allocator.cc
@@ -24,12 +24,18 @@
 namespace accounting {
 
 void* RegisterGcAllocation(size_t bytes) {
-  Runtime::Current()->GetHeap()->RegisterGCAllocation(bytes);
+  gc::Heap* heap = Runtime::Current()->GetHeap();
+  if (heap != nullptr) {
+    heap->RegisterGCAllocation(bytes);
+  }
   return malloc(bytes);
 }
 
 void RegisterGcDeallocation(void* p, size_t bytes) {
-  Runtime::Current()->GetHeap()->RegisterGCDeAllocation(bytes);
+  gc::Heap* heap = Runtime::Current()->GetHeap();
+  if (heap != nullptr) {
+    heap->RegisterGCDeAllocation(bytes);
+  }
   free(p);
 }
 
diff --git a/runtime/gc/accounting/gc_allocator.h b/runtime/gc/accounting/gc_allocator.h
index 1d96112..d4142f8 100644
--- a/runtime/gc/accounting/gc_allocator.h
+++ b/runtime/gc/accounting/gc_allocator.h
@@ -30,7 +30,7 @@
 void* RegisterGcAllocation(size_t bytes);
 void RegisterGcDeallocation(void* p, size_t bytes);
 
-static const bool kMeasureGcMemoryOverhead = false;
+static constexpr bool kMeasureGcMemoryOverhead = false;
 
 template <typename T>
 class GcAllocatorImpl : public std::allocator<T> {