nat: Check the con->imsi for the tracked IMSI

On DT messages we directly write into the tracked SCCP
connection. This means "imsi" will always be NULL at
this check. Change the code to use con->imsi

Fixes: Coverity CID 1293151
1 file changed
tree: 278d85b06b5b587858d9146d43238efe780b6fdd
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. .gitignore
  7. README