libext2fs: add sanity check for an invalid itable_used value in inode scan code

libext2fs: add sanity check for an invalid itable_used value in inode scan code

If the number of unused inodes is greater than number of inodes a
block group, this can cause an e2fsck -n run of the file system to
crash.

We should add more checks to e2fsck to detect this case directly, but
this will at least protect progams (tune2fs, dump, etc.) which use the
inode_scan abstraction from crashing on an invalid file system.

Addresses-Debian-Bug: #773795

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