qemu/queue.h: clear linked list pointers on remove

qemu/queue.h: clear linked list pointers on remove

Do not leave stale linked list pointers around after removal.  It's
safer to set them to NULL so that use-after-removal results in an
immediate segfault.

The RCU queue removal macros are unchanged since nodes may still be
traversed after removal.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200224103406.1894923-2-stefanha@redhat.com
Message-Id: <20200224103406.1894923-2-stefanha@redhat.com>