[#392] NPE during polling when one of the PRs has no source repository (#393)

In case when old PR removed it's repository NPE may occur here

Author: spyrkob

.idea/encodings.xml

@@ -73,7 +73,7 @@ public GitHubPRPullRequest(GHPullRequest pr) {
         GHCommitPointer prHead = pr.getHead();
         headSha = prHead.getSha();
         headRef = prHead.getRef();
-        sourceRepoOwner = prHead.getRepository().getOwnerName();
+        sourceRepoOwner = prHead.getRepository() == null ? null : prHead.getRepository().getOwnerName();
 
         title = pr.getTitle();
         baseRef = pr.getBase().getRef();