backtrace supression condition was wrong; fix #2472

src/backtrace.c

@@ -147,7 +147,7 @@ exc_output_backtrace(mrb_state *mrb, struct RObject *exc, output_stream_func fun
 void
 mrb_print_backtrace(mrb_state *mrb)
 {
-  if (mrb->exc || mrb_obj_is_kind_of(mrb, mrb_obj_value(mrb->exc), E_SYSSTACK_ERROR)) {
+  if (!mrb->exc || mrb_obj_is_kind_of(mrb, mrb_obj_value(mrb->exc), E_SYSSTACK_ERROR)) {
     return;
   }
   exc_output_backtrace(mrb, mrb->exc, print_backtrace_i, (void*)stderr);