Validate allocations against mappings

Bug 120032857 is seeing what appears to be allocations with incorrect
end addresses, leading to a much later crash when it tries to map
a zero page outside the valid virtual address space.  Detect allocations
that extend outside the highest or lowest memory mapping and crash
immediately instead.

Test: memunreachable_test
Bug: 120032857
Change-Id: I9be670a025143e7078360a6bf7a83219279614d9