Handle NULL entries in gmirror disk ds_bios arrays.

Entries may be removed and freed if an I/O error occurs during mirror
synchronization, so we cannot assume that all entries of ds_bios are
valid.

Also ensure that a synchronization BIO's array index is preserved after
a successful write.

Reported and tested by: pho
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon