Avoid double free() of env stack; fix #3860

Should turn on `MRB_ENV_STACK_UNSHARED` flag only after env stack
reallocation.  `malloc()` may fail.

src/vm.c

@@ -283,12 +283,12 @@ mrb_env_unshare(mrb_state *mrb, struct REnv *e)
 
     if (!MRB_ENV_STACK_SHARED_P(e)) return;
     if (e->cxt != mrb->c) return;
-    MRB_ENV_UNSHARE_STACK(e);
     p = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value)*len);
     if (len > 0) {
       stack_copy(p, e->stack, len);
     }
     e->stack = p;
+    MRB_ENV_UNSHARE_STACK(e);
     mrb_write_barrier(mrb, (struct RBasic *)e);
   }
 }