Fix crash on right click dive trip

A null pointer dereference occured after right click on a dive trip
because updateDiveInfo was called with dive == -1 causing get_dive(int)
to return null.

Wrap to avoid crash and clear dive info widget text labels.

[Dirk Hohndel: this is different from the fix I had committed earlier;
               I decided to combine the ideas, clean this one up a bit
	       more and this is the result]

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>