USSD: fix handle_ussd(): do not free() unconditionally

An internal handler may want to continue session, e.g. to request
more information from the MS.  Let's make the handlers responsible
for session state management, and check that state before calling
ss_session_free(), so a session can remain alive.

Before this patch ss->state was not set/used at all...

Change-Id: I49262e7fe26f29dedbf126087cfb8f3bb3c548dc
1 file changed