Fix "could not load message" error on valid empty message body (#1489228)

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix "could not load message" error on valid empty message body (#1489228)
 - Check filetype detection in installer and update script (#1489193)
 - Fix handling of message/rfc822 attachments on message forward and edit (#1489214)
 - Fix folder names truncation in Classic skin (#1489220)

program/lib/Roundcube/rcube_imap_generic.php

@@ -2543,7 +2543,11 @@ class rcube_imap_generic
                 $prev  = '';
                 $found = true;
 
-                while ($bytes > 0) {
+                // empty body
+                if (!$bytes) {
+                    $result = '';
+                }
+                else while ($bytes > 0) {
                     $line = $this->readLine(8192);
 
                     if ($line === NULL) {