MXS-1165 MaxInfo endless loop

If one queried in MySQL MaxInfo 'show sessions' or 'show clients',
MaxScale would go in an endless loop and finally crash when memory
ran out. The reason is the rather confusing callback-based dcb-
iterating code. Adding one line fixes the issue, although the iteration
is still rather awkward with calling dcb_foreach for every session/
client.

server/core/session.c

@@ -769,6 +769,7 @@ sessionRowCallback(RESULTSET *set, void *data)
     SESSIONFILTER *cbdata = (SESSIONFILTER*)data;
     RESULT_ROW *row = NULL;
 
+    cbdata->current = 0;
     dcb_foreach(dcb_iter_cb, cbdata);
 
     if (cbdata->row)