Fix test for pos greater than text length

If the pos is greater than the text length in runes, it will cause a
crash in refresh. This typo was introduced in the original version of
PromptWithSuggestion.

Fixes #106

line.go

@@ -596,7 +596,7 @@ func (s *State) Prompt(prompt string) (string, error) {
 
 // PromptWithSuggestion displays prompt and an editable text with cursor at
 // given position. The cursor will be set to the end of the line if given position
-// is negative or greater than length of text. Returns a line of user input, not
+// is negative or greater than length of text (in runes). Returns a line of user input, not
 // including a trailing newline character. An io.EOF error is returned if the user
 // signals end-of-file by pressing Ctrl-D.
 func (s *State) PromptWithSuggestion(prompt string, text string, pos int) (string, error) {
@@ -631,8 +631,8 @@ func (s *State) PromptWithSuggestion(prompt string, text string, pos int) (strin
 
 	defer s.stopPrompt()
 
-	if pos < 0 || len(text) < pos {
-		pos = len(text)
+	if pos < 0 || len(line) < pos {
+		pos = len(line)
 	}
 	if len(line) > 0 {
 		err := s.refresh(p, line, pos)