Fix lazy initialization of non-volatile static field in GerritLauncher

The `myArchive` static member of GerritLauncher is lazy initialized in
the `getDistributionArchive` method.

According to FindBugs:

   Because the compiler or processor may reorder instructions, threads
   are not guaranteed to see a completely initialized object, if the
   method can be called by multiple threads.

Fix this by declaring the member as 'volatile'.

Change-Id: I2774cc12c2ee749331f72ff83c3d317b073787f3

gerrit-launcher/src/main/java/com/google/gerrit/launcher/GerritLauncher.java

diff --git a/gerrit-launcher/src/main/java/com/google/gerrit/launcher/GerritLauncher.java b/gerrit-launcher/src/main/java/com/google/gerrit/launcher/GerritLauncher.java
index d49c6c7..afe61d2 100644
--- a/gerrit-launcher/src/main/java/com/google/gerrit/launcher/GerritLauncher.java
+++ b/gerrit-launcher/src/main/java/com/google/gerrit/launcher/GerritLauncher.java
@@ -299,7 +299,7 @@
     return name;
   }
 
-  private static File myArchive;
+  private volatile static File myArchive;
 
   /**
    * Locate the JAR/WAR file we were launched from.