bug joining arrows: connection points must be distinct

[Neil Zanella <nzanella cs mun ca>]

Hello,

I had an arrow connected to a connection point and was moving it around.
Accidentally the head and tail of the arrow ended up sticking to the very
same connection point which resulted in not being able to remove the
arrow from the connection point so the whole thing had to be deleted.
I would suggest that dia checks that start and end connection points
for lines be distinct to avoid this truly subtle bug.

Thanks,

Neil