resize2fs: don't exit if shrinking sparse_super2 fs to one bg

If we're shrinking a sparse_super2 filesystem to a single block group,
the superblock will be in block 0.  This is perfectly valid (for block
group 0 with a blocksize > 1024) so don't exit.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

resize/resize2fs.c

@@ -2126,7 +2126,7 @@ static errcode_t reserve_sparse_super2_last_group(ext2_resize_t rfs,
 	if (retval)
 		return retval;
 
-	if (!sb) {
+	if (last_bg && !sb) {
 		fputs(_("Should never happen!  No sb in last super_sparse bg?\n"),
 		      stderr);
 		exit(1);