Fail-fast in haskell-mode-jump-to-tag

`haskell-mode-jump-to-tag` will now crash if given a non-`nil`
non-string (inside `haskell-string-trim`), as gracjan requested in #603.

Author: antalsz

haskell.el

@@ -312,7 +312,7 @@
   (let ((ident (haskell-ident-at-point))
         (tags-file-name (haskell-session-tags-filename (haskell-session)))
         (tags-revert-without-query t))
-    (when (and (stringp ident) (not (string= "" (haskell-string-trim ident))))
+    (when (and ident (not (string= "" (haskell-string-trim ident))))
       (cond ((file-exists-p tags-file-name)
              (find-tag ident next-p))
             (t (haskell-process-generate-tags ident))))))