Don't overwrite the end pressure with sample data if one already exists

Don't overwrite the end pressure with sample data if one already exists

If we have en explicit end pressure in the dive information, we should
not change it just because we also have some samples.  The sample data
may not be complete (read: "Linus wireless connection dropped during the
dive again, and he fixed up the end pressure manually afterwards").

The beginning pressure already works correctly, because it will only use
the sample data for the first sample if no pressure existed before.

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