MFC r209683 by imp:

MFC r209683 by imp:
Add a safety-belt.  If the identified disk has 0 blocks, don't attach
it.  This can happen in some cases when plugging in SD/SmartCard PC
Cards with empty slots.  It is better to detect this bogosity, and
refuse to attach rather than panic with a division by zero (in one of
many places) down stream.

git-svn-id: svn://svn.freebsd.org/base/stable/8@215511 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f