Do not reset input ring if one exists

Fixes: debbugs:18599

lisp/erc/ChangeLog

@@ -1,3 +1,7 @@
+2014-10-02  Kelvin White  <kwhite@gnu.org>
+
+	* erc-ring.el (erc-input-ring-setup): Fixes Bug #18599
+
 2014-09-30  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* erc-track.el (erc-modified-channels-display): Update all mode lines

lisp/erc/erc-ring.el

@@ -67,7 +67,8 @@ variable.")
 (defun erc-input-ring-setup ()
   "Do the setup required so that we can use comint style input rings.
 Call this function when setting up the mode."
-  (setq erc-input-ring (make-ring comint-input-ring-size))
+  (unless (ring-p erc-input-ring)
+    (setq erc-input-ring (make-ring comint-input-ring-size)))
   (setq erc-input-ring-index nil))
 
 (defun erc-add-to-input-ring (s)