Fix bug in text selection code causing an error if the user attempts to drag a selection endpoint before any text has been selected.
Fix bug in text selection code causing an error if the user attempts to
drag a selection endpoint before any text has been selected.

(Previous commit to CLX backend had nothing to do with this, sorry)

informatimago/mcclim,quek/mcclim,quek/mcclim,informatimago/mcclim