Description:
------------
When solr server returns 500 error with no msg element, and a trace instead.
the extension segfaults instead of throwing a SolrServer Exception with code and the trace.

Only if the client is using xml response writer

Test script:
---------------
require_once "bootstrap.inc";
$options = array
(
		'hostname' => SOLR_SERVER_HOSTNAME,
		'login'    => SOLR_SERVER_USERNAME,
		'password' => SOLR_SERVER_PASSWORD,
		'port'     => SOLR_SERVER_PORT,
		'path'	   => SOLR_SERVER_PATH,
		'wt'	   => 'xml'
);

$client = new SolrClient($options);

$query = new SolrQuery('*:*');

$func = new SolrCollapseFunction('manu_id_s');

$func->setMax('sum(cscore(),field(manu_id_s))');
$func->setSize(100);
$func->setNullPolicy(SolrCollapseFunction::NULLPOLICY_EXPAND);

$query->collapse($func);
try {
	$queryResponse = $client->query($query);
} catch (SolrServerException $e) {
	printf("Exception code %d", $e->getCode());
	assert(strlen($e->getMessage()) > 0, 'Exception message is empty');
}


Expected result:
----------------
Exception code 500

Actual result:
--------------
Segmentation Fault