Initialize sub_nodes glist in the root config node.

An empty config file will result in NULL sub_nodes list rather an an
empty sub_nodes list resulting in a segfault later.

Also added a check to make sure we don't add a NULL entry to sub_nodes
list when all blocks are empty.

Reported-by: William Allen Simpson <william.allen.simpson@gmail.com>
Signed-off-by: Malahal Naineni <malahal@us.ibm.com>