Fix a issue with affect_rows in CI_DB_pdo_driver::_execute()

In case of SELECT queries PDOStatement::rowCount doesn't work as expected. This commit
makes affect_rows be initialized properly.

Signed-off-by: Rogerio Prado de Jesus <rogeriopradoj@gmail.com>
diff --git a/system/database/drivers/pdo/pdo_driver.php b/system/database/drivers/pdo/pdo_driver.php
index c38b79c..9520168 100644
--- a/system/database/drivers/pdo/pdo_driver.php
+++ b/system/database/drivers/pdo/pdo_driver.php
@@ -189,11 +189,20 @@
 	function _execute($sql)
 	{
 		$sql = $this->_prep_query($sql);
-		$result_id = $this->conn_id->query($sql);
+		$result_id = $this->conn_id->prepare($sql);
+		$result_id->execute();
 		
 		if (is_object($result_id))
 		{
-			$this->affect_rows = $result_id->rowCount();
+			if (is_numeric(stripos($sql, 'SELECT')))
+			{
+				$this->affect_rows = count($result_id->fetchAll());
+				$result_id->execute();
+			}
+			else
+			{
+				$this->affect_rows = $result_id->rowCount();
+			}
 		}
 		else
 		{
diff --git a/user_guide/changelog.html b/user_guide/changelog.html
index 4e332a0..e1a87d9 100644
--- a/user_guide/changelog.html
+++ b/user_guide/changelog.html
@@ -87,6 +87,7 @@
 	<li>Fixed a bug - When database caching was enabled, $this->db->query() checked the cache before binding variables which resulted in cached queries never being found.</li>
 	<li>Fixed a bug - CSRF cookie value was allowed to be any (non-empty) string before being written to the output, making code injection a risk.</li>
 	<li>Fixed a bug (#726) - PDO put a 'dbname' argument in it's connection string regardless of the database platform in use, which made it impossible to use SQLite.</li>
+	<li>Fixed a bug - CI_DB_pdo_driver::affect_row was not being initialized properly with SELECT queries, cause it was relying on PDOStatement::rowCount().</li>
 </ul>