of-config: fix a crash on get-config startup

2013-11-28 16:19:26 =CRASH REPORT====
  crasher:
    initial call: linc_ofconfig:init/1
    pid: <0.638.0>
    registered_name: linc_ofconfig
    exception exit: {{function_clause,[{of_config_encoder,'-simple_form/1-fun-1-',[false],[{file,"src/of_config_encoder.erl"},{line,250}]},{lists,map,2,[{file,"lists.erl"},{line,1224}]},{lists,map,2,[{file,"lists.erl"},{line,1224}]},{of_config_encoder,simple_form,1,[{file,"src/of_config_encoder.erl"},{line,250}]},{lists,map,2,[{file,"lists.erl"},{line,1224}]},{of_config_encoder,element,3,[{file,"src/of_config_encoder.erl"},{line,317}]},{of_config_encoder,simple_form,1,[{file,"src/of_config_encoder.erl"},{line,70}]},{linc_ofconfig,handle_call,3,[{file,"src/linc_ofconfig.erl"},{line,414}]}]},[{gen_server,terminate,6,[{file,"gen_server.erl"},{line,747}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}