rb_gc_impl_malloc can return NULL

Let there be rooms for each GC implementations how to handle multi
threaded situations.  They can be totally reentrant, or can have
their own mutex, or can rely on rb_thread_call_with_gvl.

In any ways the allocator (has been, but now officially is)
expected to run properly without a GVL.  This means there need be
a way for them to inform the interpreter about their allocation
failures, without relying on raising exceptions.

Let them do so by returning NULL.

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