bssgp: Handle btcx == NULL in gprs_bssgp_pcu_rx_sign (Coverity)

Currently it is assumed, that btcx is non-NULL. The btcx is only used
to obtain the BVCI in some log messages.

This commit changes that by using -1 as shown BVCI value.

Fixes: Coverity CID 1040961
Sponsored-by: On-Waves ehf
1 file changed