libblkid: Fix out of bounds reads in BEFS handling

libblkid: Fix out of bounds reads in BEFS handling

The BEFS prober is quite trusting of whatever data is fed to it and
performs almost no bounds checks. There don't seem to be any
out-of-bounds writes as far as I can tell, but there are many ways a
corrupted image could cause libblkid to read OOB and segfault, or hang
in an infinite loop.

This fix makes a few sanity-checks of the superblock, add bounds checks
wherever they seem needed, and crudely checks for cycles in the B+ tree.

Signed-off-by: Alden Tondettar <alden.tondettar@gmail.com>