squash4: fix handling of fragments and sparse files

squash4: fix handling of fragments and sparse files

1. Do not assume block list and fragment are mutually exclusive. Squash
can pack file tail as fragment (unless -no-fragments is specified); so
check read offset and read either from block list or from fragments as
appropriate.

2. Support sparse files with zero blocks.

3. Fix fragment read - frag.offset is absolute fragment position,
not offset relative to ino.chunk.

Reported and tested by Carlo Caione <carlo@endlessm.com>