Don't crash on rabbitmq_plugins_dirs fact if rabbitmqctl is not present

Before trying to access the contents of a regexp match, see if the
match was successful. On machines without rabbitmqctl this match will
return `nil`, so we can't index it with `[1]`. When that's the case,
the whole rabbitmq_plugins_dirs fact value will now be `nil`, instead
of throwing an exception.

Fixes: voxpupuli#783

lib/facter/rabbitmq_plugins_dirs.rb

@@ -2,8 +2,8 @@
   setcode do
     if Facter::Util::Resolution.which('rabbitmqctl')
       rabbitmq_pluginsdirs_env = Facter::Core::Execution.execute("rabbitmqctl eval 'application:get_env(rabbit, plugins_dir).'")
-      rabbitmq_plugins_dirs = %r{^\{ok\,\"(\/.+\/\w+)}.match(rabbitmq_pluginsdirs_env)[1]
-      rabbitmq_plugins_dirs.split(':')
+      rabbitmq_plugins_dirs_match = %r{^\{ok\,\"(\/.+\/\w+)}.match(rabbitmq_pluginsdirs_env)
+      rabbitmq_plugins_dirs_match[1].split(':') if rabbitmq_plugins_dirs_match
     end
   end
 end