e2fsck: added sanity check for xattr validation

e2fsck: added sanity check for xattr validation

Add an extra validity test in check_ext_attr().  If an attribute's
e_value_size is zero the current code does not allocate a region for it
and as a result the e_value_offs value is not verified.  However, if
e_value_offs is very large then the later call to
ext2fs_ext_attr_hash_entry() can dereference bad memory and crash
e2fsck.

Signed-off-by: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: Jim Garlick <garlick@llnl.gov>