Check if the exception is frozen; fix #4025

`exc_debug_info()` and `mrb_keep_backtrace()` raise `FrozenError`
if the exception is frozen and lead to infinite loop.

src/error.c

@@ -233,7 +233,7 @@ mrb_exc_set(mrb_state *mrb, mrb_value exc)
         (struct RBasic*)mrb->exc == mrb->gc.arena[mrb->gc.arena_idx-1]) {
       mrb->gc.arena_idx--;
     }
-    if (!mrb->gc.out_of_memory) {
+    if (!mrb->gc.out_of_memory && !MRB_FROZEN_P(mrb->exc)) {
       exc_debug_info(mrb, mrb->exc);
       mrb_keep_backtrace(mrb, exc);
     }