Deal with the first token being "trailing whitespace"

If the token is considered "trailing whitespace" and needs to be
deleted, it tries to remove the item from the linked list. However, that
doesn't work on the first and last entries. There already is a clause
for the last item, just not the first item.

Author: ekohl

lib/puppet-lint/plugins/check_whitespace/trailing_whitespace.rb

@@ -30,7 +30,7 @@ def fix(problem)
 
     prev_token = problem[:token].prev_token
     next_token = problem[:token].next_token
-    prev_token.next_token = next_token
+    prev_token.next_token = next_token unless prev_token.nil?
     next_token.prev_token = prev_token unless next_token.nil?
     tokens.delete(problem[:token])
   end