in_sql: add nil guard in shutdown

`@thread` is assigned only on the last line of `#start`,
 so any shutdown that runs on an instance whose `#start` never reached that line calls `@thread.join` on nil and raises "undefined method `join' for nil:NilClass".

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/plugin/in_sql.rb

@@ -248,8 +248,10 @@ def start
 
     def shutdown
       @stop_flag = true
-      $log.debug "Waiting for thread to finish"
-      @thread.join
+      if @thread
+        $log.debug "Waiting for thread to finish"
+        @thread.join
+      end
     end
 
     def thread_main