libext2fs: always refuse to open a file system with a zero s_desc_size

libext2fs: always refuse to open a file system with a zero s_desc_size

Commit 42c11edd0863 ("ext2fs_open[2](), return an error if s_desc_size
is too large") added a check for an insanely large s_desc_size to
prevent some failures triggered by fuzz testing.  However, it would
allow e2fsck to fall back to recover the file system by using the
backup superblocks by having e2fsck pass the flag
EXT2_FLAG_IGNORE_SB_ERRORS.  But by allowing an s_desc_Size of zero,
it's possible that e2fsck will die with a division of zero error.
With this fix, e2fsck will now print an error message and exit
instead.

https://github.com/tytso/e2fsprogs/issues/183

Fixes: 42c11edd0863 ("ext2fs_open[2](), return an error if s_desc_size is too large")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>