Fix NPE if change does not exist

If for any reason a change does not exist in the database, a NPE was
thrown when trying to get the list of patch sets.

Change-Id: Id8eec2e3e03652a0db68b85a8191065b570a5e0e
diff --git a/owners-autoassign/src/main/java/com/vmware/gerrit/owners/common/GitRefListener.java b/owners-autoassign/src/main/java/com/vmware/gerrit/owners/common/GitRefListener.java
index 67fdb1e..fa0a19a 100644
--- a/owners-autoassign/src/main/java/com/vmware/gerrit/owners/common/GitRefListener.java
+++ b/owners-autoassign/src/main/java/com/vmware/gerrit/owners/common/GitRefListener.java
@@ -94,6 +94,9 @@
       // same local thread and thus cannot be closed in this event listener.
       try {
         Change change = reviewDb.changes().get(id);
+        if (change == null) {
+          return;
+        }
         PatchList patchList = getPatchList(event, change);
         if (patchList != null) {
           PathOwners owners =