Fix coverity defects: CID 155928

CID 155928: Integer handling issues (DIVIDE_BY_ZERO)

In the current vdev label, the leaf count is always non-zero
but it doesn't hurt to check the count for future proofing.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@intel.com>
Closes #5749