[scudo] Allow Scudo allocator to return null.

POSIX allows null to be returned from some conditions that Scudo, by
default, throws an error on.

We probably want Scudo's behavior eventually, but to make things line up
nicely with extant unit tests for now (posix_memalign.c), let's use
allocator_may_return_null to allow that behavior.

Change-Id: I4f86690083d29061ef2b0e54b77be5835569e074
1 file changed