Changes

gsm0911_gsup_rx(): invalidate vsub pointer

Calling vlr_subscr_put() in gsm0911_gsup_rx() may result in free()ing
the memory pointed by vsub, so let's set it to NULL to prevent
potential use-after-free.

Change-Id: If81df71a52e4fa972b48c61d1ed1361c6629ae94