WT-1991: clear the returned WT_CURSOR reference if we closed the cursor,

there's no additional work for our caller to do.

src/session/session_api.c

@@ -314,8 +314,10 @@ __wt_open_cursor(WT_SESSION_IMPL *session,
 	 * copied.
 	 */
 	if ((*cursorp)->uri == NULL &&
-	    (ret = __wt_strdup(session, uri, &(*cursorp)->uri)) != 0)
+	    (ret = __wt_strdup(session, uri, &(*cursorp)->uri)) != 0) {
 		WT_TRET((*cursorp)->close(*cursorp));
+		*cursorp = NULL;
+	}
 
 	return (ret);
 }