Fix panic in autoclosing (#21482)

Closes #14961

Release Notes:

- Fixed a panic when backspacing at the start of a buffer with
`always_treat_brackets_as_autoclosed` enabled.

crates/editor/src/editor.rs

@@ -4142,8 +4142,10 @@ impl Editor {
 
                         if buffer.contains_str_at(selection.start, &pair.end) {
                             let pair_start_len = pair.start.len();
-                            if buffer.contains_str_at(selection.start - pair_start_len, &pair.start)
-                            {
+                            if buffer.contains_str_at(
+                                selection.start.saturating_sub(pair_start_len),
+                                &pair.start,
+                            ) {
                                 selection.start -= pair_start_len;
                                 selection.end += pair.end.len();