[PATCH] prevent segfaults on cleanup

[Erik van der Kouwe]

Dear all,

While shutting down, update_map may be called for the MSR and IO bitmaps
after those have already been freed. This patch detects this to prevent
this to avoid segmentation faults. It also prevents a segfault from
using vm->shdw_impl.current_impl when it is NULL on cleanup.

With kind regards,
Erik