[Bug #21357] Fix crash in Hash#merge with block

Prior to 49b306ecb9
the `optional_arg` passed from `rb_hash_update_block_i` to `tbl_update`
was a hash value (i.e. a VALUE). After that commit it changed to an
`update_call_args`.

If the block sets or changes the value, `tbl_update_modify` will set the
`arg.value` back to an actual value and we won't crash. But in the case
where the block returns the original value we end up calling
`RB_OBJ_WRITTEN` with the `update_call_args` which is not expected and
may crash.

`arg.value` appears to only be used to pass to `RB_OBJ_WRITTEN` (others
who need the `update_call_args` get it from `arg.arg`), so I don't think
it needs to be set to anything upfront. And `tbl_update_modify` will set
the `arg.value` in the cases we need the write barrier.

hash.c

@@ -1723,14 +1723,14 @@ tbl_update(VALUE hash, VALUE key, tbl_update_func func, st_data_t optional_arg)
         .func = func,
         .hash = hash,
         .key  = key,
-        .value = (VALUE)optional_arg,
+        .value = 0
     };
 
     int ret = rb_hash_stlike_update(hash, key, tbl_update_modify, (st_data_t)&arg);
 
     /* write barrier */
     RB_OBJ_WRITTEN(hash, Qundef, arg.key);
-    RB_OBJ_WRITTEN(hash, Qundef, arg.value);
+    if (arg.value) RB_OBJ_WRITTEN(hash, Qundef, arg.value);
 
     return ret;
 }

test/ruby/test_hash.rb

@@ -2355,6 +2355,11 @@ class TestHashOnly < Test::Unit::TestCase
     end
   end
 
+  def test_bug_21357
+    h = {x: []}.merge(x: nil) { |_k, v1, _v2| v1 }
+    assert_equal({x: []}, h)
+  end
+
   def test_any_hash_fixable
     20.times do
       assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")