Out of bound reads in chunk iterator

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11865

There is a read_unaligned of a pointer offseted by index + 1 whose corresponding slice is only valid up to index - X, where X depends on the iterators' offset.

[alamb]

Comment from Jörn Horstmann(jhorstmann) @ 2021-03-15T14:01:41.332+0000:

Good catch, this really looks suspect now. I think that second read could be replaced with reading a single byte since the bit_offset is guaranteed to be in 0..8, meaning we need at most one byte to fill in the remaining bits.