libdivecomputer does crazy gas mixes too

libdivecomputer does crazy gas mixes too

Did I just say "In comparison, the libdivecomputer output is nice and
sane"?

It turns out that libdivecomputer has been doing some drugs too when it
comes to gas mixes.  Like showing O2 percentages as 255.0% and N2
percentages as -155.0%.

Clearly libdivecomputer uses a 'unsigned char' for oxygen percentage,
and makes "-1" be "undefined".  And then it prints that non-existing mix
out, and in the process does MATH on the damn thing ("100-O2") to
"calculate" the nitrogen percentage.

Christ.

Just make the parser silently ignore the craziness, because printing out
"Strange percentage reading -155.0" a few hundred times just doesn't
make anything any better.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>