init: ensure that tmp dir exists before extracting plugins

There was wrong assumpion in '--list-plugins' implementation tmp dir
was created before we start extracting plugins to obtain metadata
about them.

Ensure the tmp dir will be created if it is not existing yet.

Change-Id: Iac29499940b3cb8255456b0dcf21e5e6e9870ecd
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginLoader.java

@@ -166,6 +166,9 @@ public class PluginLoader implements LifecycleListener {
 
   public static File storeInTemp(String pluginName, InputStream in,
       SitePaths sitePaths) throws IOException {
+    if (!sitePaths.tmp_dir.exists()) {
+      sitePaths.tmp_dir.mkdirs();
+    }
     return asTemp(in, tempNameFor(pluginName), ".jar", sitePaths.tmp_dir);
   }