widget: fix Editor panic

If you created an Editor and immediately SetCaret, it panicked because
e.lines was nil and it looked at e.lines[0].

- Add e.makeValid at the top of SetCaret.
- Add a test case for this situation.

Signed-off-by: Larry Clapp <larry@theclapp.org>

widget/editor.go

@@ -1186,6 +1186,7 @@ func (e *Editor) Selection() (start, end int) {
 // and end are in bytes, and represent offsets into the editor text. start and
 // end must be at a rune boundary.
 func (e *Editor) SetCaret(start, end int) {
+	e.makeValid()
 	// Constrain start and end to [0, e.Len()].
 	l := e.Len()
 	start = max(min(start, l), 0)