When exiting the loop, stopidx is 0, which means that if there
are no stoplevels, stoplevels[stopidx + 1] generates an
out-of-bounds access. Instead, suppose a stop at 3m or 10ft.
Suggested-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>