Fix Segmentation fault when trying to print an empty plot.

Fix Segmentation fault when trying to print an empty plot.

When printing an empty plot, the function was missing nullability check for 'current_dive'. Now the print of an empty plot results with an empty blank page.
A better solution could be making unsensitive the Print entry in the menu, until a plot is loaded.

Signed-off-by: Riccardo Albertini <ssirowain@gmail.com>