[sgen] Don't return a pointer from the out-of-memory function.

[sgen] Don't return a pointer from the out-of-memory function.

The pointer the function would have to return would be to a chunk of managed
memory, for the object to be allocated.  It can't do that because it is not in charge
of the managed heap.  The best it could do would be to free some unmanaged
memory and tell the allocator try again.