e2fsck: check for zero length extent

e2fsck: check for zero length extent

If an extent has e_len set to zero, the kernel will oops with a
BUG_ON.  Unfortunately, e2fsck wasn't catching this case.  The kernel
needs to be fixed to notice this case and call ext4_error() instead of
failing an assertion check, but e2fsck should catch this case and
repair it (by deleting the errant extent).

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>