Don't access invalid mem in totemconfig interfaces

When ringnumber in config file was set to value bigger or equal to
INTERFACE_MAX, we are using this big value as index to totemconfig
interfaces array, resulting to access to invalid memory and segfault.

Instead of that, ringnumber is now checked and proper error message is
printed if value is too big.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>