api helpers: Save/restore more values in try_enter/try_leave

This fixes memory leak reported by ASAN. This also somehow fixes test40, though 
I have no idea why except that that test yields memory leak report.

src/nvim/ex_eval.c

@@ -565,6 +565,8 @@ static void discard_exception(except_T *excp, int was_finished)
 void discard_current_exception(void)
 {
   discard_exception(current_exception, FALSE);
+  // Note: all globals manipulated here should be saved/restored in
+  // try_enter/try_leave.
   current_exception = NULL;
   did_throw = FALSE;
   need_rethrow = FALSE;