Fix incorrect uses of G1CollectedHeap::heap_region_containing()

G1CollectedHeap::heap_region_containing() currently returns HeapRegion* of uncommitted heap regions which it should actually error out on.
At least some callers (marking code, verification code) explicitly check for nullptr on return which suggests that the intention has been to use G1CollectedHeap::heap_region_containing_or_null() instead.

That G1CollectedHeap::heap_region_containing_or_null() method explicitly mentions that it would return nullptr on uncommitted regions.