Editing existing line-annotations accidentally creates multi-lines

[jaagut]

A line can be created by creating one point and moving the cursor outside of the image, then pressing V to save.
Afterwards editing this line accidentally creates an multi-line, which can be saved successfully. This should not be allowed and saving the multi-line should be prevented by the server.

[jaagut]

This bug happens mostly by drawing a line outside of the image, but this bug also happens (sometimes) by creating both points of the line, saving it and afterwards editing this line.