Fix regression in forward-single-char

This crashes if the autosuggesion is exhausted.  C++ used

    autosuggestion.text.substr(pos, 1)

which throws if pos is OOB but not if pos + 1 is.

src/reader.rs

@@ -3944,12 +3944,14 @@ fn accept_autosuggestion(
                 self.autosuggestion.text.clone(),
             );
         } else if single {
-            self.replace_substring(
+            let pos = self.command_line.len();
-                EditableLineTag::Commandline,
+            if pos + 1 < self.autosuggestion.text.len() {
-                self.command_line.len()..self.command_line.len(),
+                self.replace_substring(
-                self.autosuggestion.text[self.command_line.len()..self.command_line.len() + 1]
+                    EditableLineTag::Commandline,
-                    .to_owned(),
+                    pos..pos,
-            );
+                    self.autosuggestion.text[pos..pos + 1].to_owned(),
+                );
+            }
         } else {
             // Accept characters according to the specified style.
             let mut state = MoveWordStateMachine::new(style);

tests/checks/tmux-complete.fish

@@ -84,3 +84,13 @@ isolated-tmux send-keys 'echo sug' C-w C-z
 tmux-sleep
 isolated-tmux capture-pane -p
 # CHECK: prompt 6> echo suggest this
+
+isolated-tmux send-keys C-u 'bind \cs forward-single-char' Enter C-l
+isolated-tmux send-keys 'echo suggest thi'
+tmux-sleep
+isolated-tmux send-keys C-s
+tmux-sleep
+isolated-tmux send-keys C-s
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 7> echo suggest this