Fix imenu with Clojure code in string or comment

Ignore error that's thrown from `down-list` when called
with point inside a string or comment.

E.g. with code like:

```
(defn foo []
  (let [a "
(defn b [_]
  (bar {:bla \"bla\"}))"]))
```

`clojure-match-next-def` calls `down-list` with point inside
the string and `down-list` will throw an user-error with
"This command doesn't work in strings or comments".

Author: dakra

clojure-mode.el

@@ -776,7 +776,8 @@ Called by `imenu--generic-function'."
       (let (found?
             (deftype (match-string 2))
             (start (point)))
-        (down-list)
+        (ignore-errors
+          (down-list))
         (forward-sexp)
         (while (not found?)
           (ignore-errors