Fix use of uninitialized variable if there are no samples

Fix use of uninitialized variable if there are no samples

When creating the plot_info, the 'entry' variable pointing to the last
plot_info data was not initialized (because there was no data to fill
in), and was then incorrectly used to fill in the last tank pressure.

We also used to look at 'dive->sample[0].cylinderindex' even if no
sample[0] necessarily existed.

Reported-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>