vlr auth: gracefully reject malformed auth response

Instead of just closing down the conn hard, actually feed invalid auth response
data to vlr_subscr_rc_auth_resp() in order to trigger all the actions we want
to see with a failed authentication:
- a GSUP signal that the auth failed,
- a LU reject.
Verify this in new test_wrong_sres_length() in msc_vlr_test_gsm_authen.c.

Note that in gsm48_rx_mm_auth_resp(), the is_r99 flag is falsely derived from
the RES length, which upcoming commit Ib7f7d89a8b9455d2c022d53d74328fa7488577f4
will fix.

Change-Id: I4179a290069ac61d0662de4ec7ca3edb76988899
3 files changed