bugfix: cut (C-x) without first selecting anything

manual_tests

@@ -36,6 +36,9 @@ persistence:
   draw a line, circle, rectangle, square, polygon, quit, restart. All the shapes you drew should still be visible.
   select a point and name it, quit, restart. Name is still visible.
 
+clipboard:
+  cut/copy without making a selection
+
 resize:
   create a file containing a long line of characters without spaces. try
   resizing the window vertically and horizontally, as far as possible.

select.lua

@@ -103,12 +103,14 @@ function Text.to_pos(x,y)
 end
 
 function Text.cut_selection()
+  if Selection1.line == nil then return end
   local result = Text.selection()
   Text.delete_selection()
   return result
 end
 
 function Text.delete_selection()
+  if Selection1.line == nil then return end
   local minl,maxl = minmax(Selection1.line, Cursor1.line)
   local before = snapshot(minl, maxl)
   Text.delete_selection_without_undo()