hlr_ussd.c: rx_proc_ss_req(): fix NULL pointer dereference

The SS payload is mandatory for GSUP PROC_SS_{REQ,RSP} messages
with session state BEGIN or CONTINUE, and optional for the END.

Make sure that it's present for both BEGIN and CONTINUE, consider
received message as incorrect otherwise. In case of the END, call
handle_ussd() / handle_ss() only if SS payload is present.

Change-Id: Ia71cabbf396bd1388e764a1749e953ac1782e307
Fixes: CID#188841
1 file changed