Because of helping, it is possible that a core thread that wants to
destroy another thread at the kernel is using the scheduling context of
the thread that shall be destroyed at this point in time. When building
without GENODE_RELEASE defined, this always triggers an assertion in the
kernel. But when building with GENODE_RELEASE defined, this might silently
lead to kernel-memory corruption. This commit eliminates the latter case.
Should be reverted as soon as the scheduler is able to remove its head.
Ref #1537
8a99c08ae4