Fix a regression bug that "equal" against nonexitent reference always matches

grn_obj_get_value() returns value its type is not real value type
for nonexistent reference in the middle of accessor.

For example: Think about the following schema:

    table_create Bookmarks TABLE_HASH_KEY ShortText
    column_create Bookmarks title COLUMN_SCALAR ShortText

    table_create Users TABLE_HASH_KEY ShortText
    column_create Users bookmark COLUMN_SCALAR Bookmarks

And the following data:

    load --table Bookmarks
    [
    {"_key": "http://groonga.org/", "title": "Groonga"}
    ]

    load --table Users
    [
    {"_key": "alice", "bookmark": "http://groonga.org/"},
    {"_key": "bob"}
    ]

"bob" user is important. "bob"'s bookmark column value is
nonexistent. grn_obj_get_value() for "bookmark.name" accessor and "bob"
user returns Bookmarks type value not ShortText type value. Because
grn_obj_get_value() for accessor stops value resolving when there is
nonexistent reference.

With this change, grn_obj_get_value() always returns real value type
even if there is nonexistent reference.