Fix up end conditions for dives

We used to have the dive plot have two "filler" entries at the beginning
and the end, and indeed that is how they are allocated.  However, we fix
up "pi->nr" later to be "lastindex+1", where "lastindex" is the index of
the time we surface.

So when we loop over the plot entries, we actually need to loop all the
way to the end: use "i < pi->nr" instead of "i < pi->nr-2".

We still do have the two extra filler entries at the beginning, though.
So depending on the loop, we might want to start at entry 2.

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