fs/squashfs: Fix Coverity Scan defects

Fix defects such as uninitialized variables and untrusted pointer
operations. Most part of the tainted variables and the related defects
actually comes from Linux's macro get_unaligned_le**, extensively used
in SquashFS code. Add sanity checks for those variables.

Signed-off-by: Joao Marcos Costa <joaomarcos.costa@bootlin.com>