Do not match on result of last_mut() for data_segments
Do not match on result of last_mut() for data_segments

It should not every be possible for data_segments to have zero entries.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

trgill/stratisd,trgill/stratisd,mulkieran/stratisd,mulkieran/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd