Fix default value for missing surface pressure from divecomputer

We should *not* default to the incorrect "1 bar".  Instead, we should
leave the resuling pressure at 0 mbar, which leaves visual entries empty
and uses the default surface pressure for calculations.

Reported-by: Pedro Neves <nevesdiver@gmail.com>
Cc: Patrick Valsecchi <patrick@thus.ch>
Cc: Dirk Hohndel <dirk@hohndel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use "rint()" instead of rounding manually with "+ 0.5"

rint() is "round to nearest integer", and does a better job than +0.5
(followed by the implicit truncation inherent in integer casting).  We
already used 'rint()' for values that could be negative (where +0.5 is
actively wrong), let's just make it consistent.

Of course, as is usual for the messy C math functions, it depends on the
current rounding mode.  But the default round-to-nearest is what we want
and use, and the functions that explicitly always round to nearest
aren't standard enough to worry about.

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