Fix squashfs failing to load sparse files

SquashFS supports sprase blocks in files - that is, if a given block is
composed only of zeros, it's not written to the output file to save
space and it's on-disk length field is set to zero to indicate that.

Previously the squashfs driver did not recognise that, and would attempt
to read and decompress a zero-sized block, which obviously failed.

The following command may be used to create a file for testing:

cat <(dd if=/dev/urandom of=/dev/stdout bs=1M count=1) \
	<(dd if=/dev/zero of=/dev/stdout bs=1M count=1) \
	<(dd if=/dev/urandom of=/dev/stdout bs=1k count=200) >test_file

Signed-off-by: Campbell Suter <campbell@snapit.group>