A way to differentiate missed allocations from profiling being disabled

[itamarst]

Sometimes we get free() of address we don't recognize. It is currently hard to distinguish the following circumstances:

1. Allocation wasn't added because tracking was off (explicitly, or due to startup).
2. Allocation was never captured due to not overriding some allocation API (valloc() or something).
3. A free() of bogus address due to a bug in application code.

One solution is to always track all allocations—but when profiling is disabled, record the allocations with size 0. As a result they won't affect outcomes of profiling, but we will at least know those allocations existed.