Fix possible data race condition

This issue was found by scan.coverity.com (CID 19961)
which is a static code analysis tool, free for open source
code.

Assuming we have two threads both can pass the !ready
condition before entering the critical section protected with
"synchronized (dataDir)". So theoretically both threads will
arrive at setting ready to true.

Change-Id: I4b833279d755b32e4b1e0817cca5584c21c6346a

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

@@ -253,6 +253,7 @@ public class ServerPlugin extends Plugin {
             public File get() {
               if (!ready) {
                 synchronized (dataDir) {
+                  if (!ready) {
                     if (!dataDir.exists() && !dataDir.mkdirs()) {
                       throw new ProvisionException(String.format(
                           "Cannot create %s for plugin %s",
@@ -261,6 +262,7 @@ public class ServerPlugin extends Plugin {
                     ready = true;
                   }
                 }
+              }
               return dataDir;
             }
           });