Prevent g_access calls to bad multipath members

When a multipath member is orphaned its access members are zeroed before its
removed if marked for wither, so prevent any future calls to g_access on
such members.

This prevents a panic on debug kernels which validates the resultant values
aren't negative.

Reviewed by: mav
MFC after: 2 weeks
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4416