epoch: possible reads of uninitialized data

Reported in servo/servo#23077

==130127== Thread 43 WRRenderBackend#:
==130127== Conditional jump or move depends on uninitialised value(s)
==130127==    at 0x95D09AA: crossbeam_epoch::internal::Global::collect (internal.rs:189)
==130127==    by 0x957E156: crossbeam_epoch::internal::Local::pin (internal.rs:379)
==130127==    by 0x9551625: crossbeam_epoch::collector::LocalHandle::pin (collector.rs:72)
==130127==    by 0x95550E7: crossbeam_epoch::default::pin::{{closure}} (default.rs:23)
==130127==    by 0x9555097: crossbeam_epoch::default::with_handle::{{closure}} (default.rs:42)
==130127==    by 0x955668D: <std::thread::local::LocalKey<T>>::try_with (local.rs:300)
==130127==    by 0x9554F72: crossbeam_epoch::default::with_handle (default.rs:42)
==130127==    by 0x63530B9: crossbeam_epoch::default::pin (default.rs:23)
==130127==    by 0x634717E: <crossbeam_channel::flavors::list::Channel<T>>::start_send (list.rs:161)
==130127==    by 0x6347DB8: <crossbeam_channel::flavors::list::Channel<T>>::send (list.rs:381)
==130127==    by 0x634E381: <crossbeam_channel::channel::Sender<T>>::send (channel.rs:416)
==130127==    by 0x634F347: compositing::compositor_thread::CompositorProxy::send (compositor_thread.rs:31)
==130127== 
==130127== Conditional jump or move depends on uninitialised value(s)
==130127==    at 0x95D4A2F: core::ptr::real_drop_in_place (ptr.rs:193)
==130127==    by 0x95D09D0: crossbeam_epoch::internal::Global::collect (internal.rs:192)
==130127==    by 0x957E156: crossbeam_epoch::internal::Local::pin (internal.rs:379)
==130127==    by 0x9551625: crossbeam_epoch::collector::LocalHandle::pin (collector.rs:72)
==130127==    by 0x95550E7: crossbeam_epoch::default::pin::{{closure}} (default.rs:23)
==130127==    by 0x9555097: crossbeam_epoch::default::with_handle::{{closure}} (default.rs:42)
==130127==    by 0x955668D: <std::thread::local::LocalKey<T>>::try_with (local.rs:300)
==130127==    by 0x9554F72: crossbeam_epoch::default::with_handle (default.rs:42)
==130127==    by 0x63530B9: crossbeam_epoch::default::pin (default.rs:23)
==130127==    by 0x634717E: <crossbeam_channel::flavors::list::Channel<T>>::start_send (list.rs:161)
==130127==    by 0x6347DB8: <crossbeam_channel::flavors::list::Channel<T>>::send (list.rs:381)
==130127==    by 0x634E381: <crossbeam_channel::channel::Sender<T>>::send (channel.rs:416)

[taiki-e]

Closing - There is no way to reproduce the original problem with the latest crossbeam, and it is very likely that the relevant code in epoch has been fixed in #458.