Fix undefined area set if grouping is not used.

[liske]

This fixes the following error if host grouping is not used:

./ospfcli2dot 
ospfcli2dot - takes the output of "show ip ospf database router" and optionally a hostfile
outputs a GraphViz DOT file corresponding to the network topology

v0.3 alpha, By Foeh Mannay, December 2017

Enter input filename: ospf.show
Enter hostnames file or press enter to continue without one: 
If you want to group by hostname, enter the separator now
 (or press enter to continue): 
Enter output filename: ospf.dot
Traceback (most recent call last):
  File "./ospfcli2dot", line 233, in <module>
    if(areas):
NameError: name 'areas' is not defined