Compare string lengths when parsing the snapshot mask

We used to rely on the result from strncmp() without comparing the length of
the strings involved. Even worse, any single-character format specifier would
enable zip-format due to the optional '.'-prefix since the length of the
mask then would become zero.

Noticed-by: Evan Martin <sys@neugierig.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>