dive merge seems broken in latest master

[Dirk Hohndel]

Berthold,

This looks like it is yours...

Bisect brings us to this:

commit 8c2383b4952fa22d41745d29484462ed6a67112b
Author: Berthold Stoeger <bstoeger at mail.tuwien.ac.at>
Date:   Sun Aug 12 22:47:07 2018 -0400

I have too much nitrogen (and too little sleep) to figure out HOW and WHY this breaks things,
but on a trip with multiple dive computers, if I try to download (or plain import) the dives from 
my second DC, Subsurface crashes. In process_imported_dive(); there appears to be garbage 
int the dive_table.dives[] and that gets dereferenced to calculate the end time of a dive.

/D
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