don't grab any key, when -key is not used

before this, the NoSymbol keysym was grabbed, which causes
a badAccess error on a second instance of dunst if both don't use
the -key option.

dunst.c

@@ -248,8 +248,10 @@ setup(void) {
 	                    CWOverrideRedirect | CWBackPixmap | CWEventMask, &wa);
 
     /* grab keys */
+    if(key != NoSymbol) {
         code = XKeysymToKeycode(dc->dpy, key);
         XGrabKey(dc->dpy, code, mask, root, True, GrabModeAsync, GrabModeAsync);
+    }
 }
 
 void