Guard against a block size of 0 in the label. When the first

Guard against a block size of 0 in the label.  When the first
superblock is invalid, fsck looks at the label to help guess where
the next superblock should be.  If the partition type is 4.2BSD,
fsck assumed that the block size was valid and divided by it, so
it dumped core if the size was 0.

Initialization of the label was broken almost 3 years ago in rev.1.9
of newfs/newfs.c.  Newfs does not change the label at all, so there
is no problem (except the breakage of the automatic search for
backup superblocks) unless something else sets the partition type
to 4.2BSD.  However, it is too easy to set partition types to
4.2.BSD by copying an old label or by using a disktab entry to
create the label.

PR:	2537