Fix incorrect checks for the fwrite() return value

! fwrite() could trigger false-positives as it is possible for it to return 0
instead of boolean FALSE. (issue #2822)

Also removed an unnecessary log level check that caused an extra space to be inserted
for the INFO level. (proposed in PR #2821)
diff --git a/system/libraries/Email.php b/system/libraries/Email.php
index 7e80ffb..9487ad4 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -2097,7 +2097,7 @@
 	 */
 	protected function _send_data($data)
 	{
-		if ( ! fwrite($this->_smtp_connect, $data.$this->newline))
+		if (fwrite($this->_smtp_connect, $data.$this->newline) === FALSE)
 		{
 			$this->_set_error_message('lang:email_smtp_data_failure', $data);
 			return FALSE;