[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.

PHPBB3-12986
Merge pull request #2881 from rxu/ticket/12986

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

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