sgsn: seleted_apn_str might be NULL do not strcpy

The subscriber might simply not be allowed to use the APN
that was specified. So selected_apn_str might very well be
NULL.

Fixes: Coverity CID#1302853
1 file changed
tree: 7b7c6e83c5ca37791c79ec67a6513e728bf47a99
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. .gitignore
  7. README