msOffsetPolyline crashes if a line with only 1 point is available.
Currently this can occur since when offsetting lines coordinates are first transformed to image-coordinates, rounded to integers and then duplicates are removed.
Short lines can therefore end of with only 1 point.
Don't know if msOffsetPolyline should handle this, or if the reduction algorithm should make sure that a line at least contains two points.
See also: #4382
msOffsetPolyline definitely can't handle this itself as there is no way to calculate an offset direction. The segfault should be corrected in this case. also see #4143 for an undocumented (as of yet) workaround, whereyou can change the simplification method use for that specific layer so the lines are not degenerated into a single point.
don't crash on ofsetting degenerate lines (#4383)