Boost doesn't handle voting on comments correctly

If boost_votingapi_insert is called but the voted item is a comment rather than a node, then a PHP error may occur. From line 1652 of boost.module:

function boost_votingapi_insert($votes) {
  if (!variable_get('boost_enabled', CACHE_NORMAL)) return;

  foreach ($votes as $vote) {
    $node = node_load($vote['content_id'], NULL, TRUE);
    boost_expire_node($node, $vote['content_id']);
  }
}

For example if the vote is applied to a comment(cid=25) of a node (nid=480), then the $vote['content_id'] is 25. The following calls to node_load and boost_expire_node are now incorrectly using the cid as an nid, rather than first finding the parent node that was commented upon (and its nid). As a result, the cache on the wrong node is expired and if the node doesn't actually exist, then a PHP error occurs.