Fix main space memory leak and add checks.

The hypothesis is that we were leaking the main space and its
bitmaps, then eventually we would run out of virtual address space,
which would cause a null bitmap (DCHECK). Finally when we tried
adding the space with a null bitmap to the heap bitmap it segfaulted.

Changed some non performance critical DCHECK -> CHECK.

Bug: 16563323

(cherry picked from commit 2796a166)

Change-Id: Ifa9d866c6c89eff22a547af4db70bc79a77690ed