[TSan][Darwin] Make malloc_size interceptor more robust

Previously we would crash in the TSan runtime if the user program passes
a pointer to `malloc_size()` that doesn't point into app memory.

In these cases, `malloc_size()` should return 0.

For ASan, we fixed a similar issue here:
https://reviews.llvm.org/D15008

Radar-Id: rdar://problem/86213149

Differential Revision: https://reviews.llvm.org/D115947