Copy Context before start in Trampoline

The CommandFactoryProvider uses a Trampoline class
to put each command into the Executor.  In doing
so it keeps a Context which the Runnable it creates
has access to in its catch block and its toString()
method.  Since the ctx variable which stores this
in the Trampoline class is nulled out in the public
destroy() method, an NPE can occur when the Runnable
is used.  I have seen an NPE on our server on the
toString() line when I repeatedly query the queue.

To avoid the ctx var from being nulled out by the
destroy method, copy it first into the Runnables
scope.

Change-Id: Ia7e6cc46c0b96dc9a3a7b4c8e606235918d3e7c1

gerrit-sshd/src/main/java/com/google/gerrit/sshd/CommandFactoryProvider.java

@@ -109,6 +109,7 @@ class CommandFactoryProvider implements Provider<CommandFactory> {
 
     public void start(final Environment env) throws IOException {
       this.env = env;
+      final Context ctx = this.ctx;
       startExecutor.execute(new Runnable() {
         public void run() {
           try {