vty: fix segfault if rf_ctl is not set

This is the case of osmo-nitb, since it does not call
osmo_bsc_rf_create().

term1: $ ./osmo-nitb -c ../openbsc.cfg.nanobts
term2: $ telnet localhost 4242
term2: OpenBSC> show network
term1: Segmentation fault
1 file changed
tree: 0216c049f6bb92cba9d700bd4dd78a99d9ad3160
  1. hlrsync/
  2. linux-kernel/
  3. openbsc/
  4. wireshark/
  5. README