[ticket/12986] Fix detection of common words in search

$search->get_common_words() call goes before the split_keywords() call,
but $search->common_words array is being generated
within the function $search->split_keywords(). Thus, get_common_words()
returned an empty array instead of common words array.

<a href="https://tracker.phpbb.com/browse/PHPBB3-12986">PHPBB3-12986</a>.