Clear divecomputer saved status before saving the nicknames to XML

In commit c7169bd24f22 "Fix nickname saving in XML file to deal with utf8
characters" I added the helper function to clear the "this divecomputer
has already been saved"-flag. But then forgot to call it from save_dives
before saving the divecomputer nicknames.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>