crossbeam-epoch's atomic.rs doesn't handle alloc::alloc returning null pointers

[erickt]

In Fuchsia, we're doing an update of our vendored version of crossbeam. As part of it, we found some uses of alloc::alloc that may be incorrect, like https://github.com/crossbeam-rs/crossbeam/blob/master/crossbeam-epoch/src/atomic.rs#L254. According to the documentation for GlobalAlloc, it's possible for allocation to return a null pointer in two cases:

• memory is exhausted
• layout doesn't match the allocator's size or alignment constraints.

This risks undefined behavior if these conditions are met.

[taiki-e]

Thanks! I filed #690 to fix this.

[taiki-e]

The fix is released in crossbeam-epoch 0.9.4.