Fix DB error when deleting message from cache (closes #1483835).

program/include/rcube_imap.inc

@@ -1434,7 +1434,7 @@ class rcube_imap
       $start_index = 100000;
       foreach ($a_uids as $uid)
         {
-        $index = array_search($uid, $a_cache_index);
+        if (($index = array_search($uid, $a_cache_index)) !== FALSE)
           $start_index = min($index, $start_index);
         }