Fix GC memory overhead accounting.

There was some missing null checks.

Bug: 16238192

Change-Id: Iaf8d752db5f21e76f668c0066a063239ff374eee
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> {