[Minor] --var doesn't work

[Minor] --var doesn't work

We can add varaibels on the command line after argument or using --var parameter.
When --var is used, an assertion is triggered:

   ./rspamd.install/bin/rspamd --var LOGDIR=/tmp/
   (rspamd:7171): GLib-CRITICAL **: g_hash_table_insert_internal: assertion 'hash_table != NULL' failed

The hash table is not initialized, because the table is initialized after
the argument parser.

This patch initialize the hash table on demand.

Note: the patch is tested on 1.9 version and not on master, because I can't compile the master branch.

Note: I suggest to backport the patch on 1.9 version