vty: Address coverity issue with subscr->name never being NULL

Coverity points out that subscr->name is an array and never NULL.
Use strlen instead in this check.

Fixes: Coverity CID 1040717
1 file changed
tree: 1609c7e436b93e27989fcccf6d65fddf7e9d900d
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README