MapLine: fix segfault for small line segments

If two adjacent waypoints have the same locations, then
a segfault occurred because the arrow head angle is
undefined.
WaypointCurve: update signal name

This was changed ages ago from MapItem* to WaypointItem*,
but this signal wasn't updated accordingly.
WaypointCurve: fix the calculation of the arrow head

This was crashing when calculating fillets due to poor handling
of short lengths.
Merge pull request #1076 from peabody124/waypoints_fix

Waypoints fix