Grab wpdb instance from Complex Query class property.

Previously, we'd only load from the parameter. The `wpdb` parameter
is nullable for Complex_Query, but not for Simple_Query, causing
a fatal error if ommitted.

Author: TimothyBJacobs

src/Query/Complex_Query.php

@@ -72,7 +72,7 @@ public function __construct( Table $table, array $args = array(), \wpdb $wpdb =
 
 		$this->table    = $table;
 		$this->wpdb     = $wpdb ?: $GLOBALS['wpdb'];
-		$this->db_query = new Simple_Query( $wpdb, $table );
+		$this->db_query = new Simple_Query( $this->wpdb, $table );
 
 		$this->args = wp_parse_args( $args, $this->get_default_args() );