gluon-mesh-vpn-core: avoid reading null pointer

In case the limit_ingress or limit_egress options are not present in
gluon's mesh_vpn section the respondd provider compares a string literal
with a NULL pointer, crashing respondd.

Check both pointers prior to comparing them in order to mitigate this
issue.

Suggested-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David Bauer <mail@david-bauer.net>