Fix crash happening from time to time

This is definitely a band-aid, and not a perfect fix, but I'm not able
to identify why this crash sometimes happens and sometimes doesn't:

F, [2014-02-10T18:44:19.682639 #22058] FATAL -- :
ActionView::TemplateError (undefined method `[]' for nil:NilClass) on line #5 of app/views/nodes/_show_barclamps.html.haml:
2:   %th
3:     = t("model.attributes.node.barclamps")
4:   %td{ :colspan => 3 }
5:     = node_barclamp_list(@node)

    app/models/service_object.rb:88:in `barclamp_category'
    /usr/lib64/ruby/1.8/net/protocol.rb:135:in `map'
    app/models/service_object.rb:86:in `each'
    app/models/service_object.rb:86:in `map'
    app/models/service_object.rb:86:in `barclamp_category'
    app/models/proposal_object.rb:131:in `category'
    app/helpers/nodes_helper.rb:377:in `node_barclamp_list'
    app/helpers/nodes_helper.rb:357:in `map'
    app/helpers/nodes_helper.rb:357:in `node_barclamp_list'
    app/helpers/nodes_helper.rb:356:in `tap'
    app/helpers/nodes_helper.rb:356:in `node_barclamp_list'
    app/views/nodes/_show_barclamps.html.haml:5:in `_run_haml_app47views47nodes47_show_barclamps46html46haml_locals_object_show_barclamps'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    haml (3.1.6) lib/haml/helpers.rb:90:in `non_haml'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    app/views/nodes/_show.html.haml:18:in `_run_haml_app47views47nodes47_show46html46haml_locals_object_show'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    haml (3.1.6) lib/haml/helpers.rb:90:in `non_haml'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    app/views/nodes/show.html.haml:6:in `_run_haml_app47views47nodes47show46html46haml'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:13:in `render'
    haml (3.1.6) rails/./lib/haml/helpers/action_view_mods.rb:13:in `render'
    app/controllers/nodes_controller.rb:310:in `show'
    sass (3.2.12) rails/./lib/sass/plugin/rack.rb:54:in `call'
    sass (3.2.12) rails/./lib/sass/plugin/rack.rb:54:in `call'
    rainbows (4.3.1) lib/rainbows/event_machine/client.rb:41:in `app_call'
    rainbows (4.3.1) lib/rainbows/event_machine/client.rb:40:in `catch'
    rainbows (4.3.1) lib/rainbows/event_machine/client.rb:40:in `app_call'
    rainbows (4.3.1) lib/rainbows/ev_core.rb:97:in `on_read'
    rainbows (4.3.1) lib/rainbows/event_machine/client.rb:25:in `receive_data'
    eventmachine (1.0.3) lib/eventmachine.rb:187:in `run_machine'
    eventmachine (1.0.3) lib/eventmachine.rb:187:in `run'
    rainbows (4.3.1) lib/rainbows/event_machine.rb:86:in `worker_loop'
    rainbows (4.3.1) lib/rainbows/http_server.rb:44:in `worker_loop'
    unicorn (4.1.0) lib/unicorn/http_server.rb:485:in `spawn_missing_workers'
    rainbows (4.3.1) lib/rainbows/http_server.rb:60:in `spawn_missing_workers'
    unicorn (4.1.0) lib/unicorn/http_server.rb:135:in `start'
    rainbows (4.3.1) bin/rainbows:122
    /usr/bin/rainbows:19:in `load'
    /usr/bin/rainbows:19
Merge pull request #928 from vuntz/fix-crash

Fix crash happening from time to time