Initialize device_data_t data

If the user never clicks on the log and dump checkboxes (and that's what
we expect to be the case by default), then the log / dump flags (and
filenames) were never initialized.

I am reasonably certain this will close the following three bugs, that all
show the same symptom: they behave as if libdivecomputer dump was set,
even if it wasn't.

Fixes: #426
Fixes: #431
Fixes: #435

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>