Revert ae4217e; fix #3619

Reopens #3550. Those 2 issues are exclusive.

Author: matz

src/gc.c

@@ -769,7 +769,7 @@ obj_free(mrb_state *mrb, struct RBasic *obj, int end)
         while (ce <= ci) {
           struct REnv *e = ci->env;
           if (e && !is_dead(&mrb->gc, e) &&
-              e->tt == MRB_TT_ENV && !MRB_ENV_STACK_SHARED_P(e)) {
+              e->tt == MRB_TT_ENV && MRB_ENV_STACK_SHARED_P(e)) {
             mrb_env_unshare(mrb, e);
           }
           ci--;