src: fix label indentation

In preparation for a cpplint version bump, move labels to be unindented.
The one space indentation we use now is flagged in cpplint 1.5.2.

Refs: https://softwareengineering.stackexchange.com/q/151806/26510

PR-URL: #36213
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

src/node_buffer.cc

@@ -619,7 +619,7 @@ void Fill(const FunctionCallbackInfo<Value>& args) {
                                     nullptr);
   }
 
- start_fill:
+start_fill:
 
   if (str_length >= fill_length)
     return;