Stop double-free of query

There is a chance that a query is executed by WordPress's WPDB before
the health checks plugin wrapper has been loaded. In this case the
previous query in WPDB would be freed twice.

This patch fixes this by allowing any previous query to be flushed.

Fixes #25

inc/hooks.php

@@ -11,6 +11,7 @@
 class MDB_DB extends wpdb
 {
 	public $total_query_time = 0.0;
+	private $first_run = true;
 
 	public function loadFromParentObj($parentObj)
 	{
@@ -22,6 +23,13 @@ public function loadFromParentObj($parentObj)
 
 	public function query($query)
 	{
+		// This covers the case where WPDB is used before this plugin is loaded.
+		// It stops the attempt to clean up a query twice.
+		if ($this->first_run) {
+			$this->first_run = false;
+			$this->result = null;
+			parent::flush();
+		}
 		$this->timer_start();
 		$result = parent::query($query);
 		$this->total_query_time += $this->timer_stop();