avoid null pointer dereference (#2291)

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/history/RCSHistoryParser.java

@@ -80,8 +80,13 @@ History parse(File file, Repository repos) throws HistoryException {
     }
 
     private History parseFile(File file) throws IOException {
+        File rcsfile = getRCSFile(file);
+        if (rcsfile == null) {
+            return null;
+        }
+
         try {
-            Archive archive = new Archive(getRCSFile(file).getPath());
+            Archive archive = new Archive(rcsfile.getPath());
             Version ver = archive.getRevisionVersion();
             Node n = archive.findNode(ver);
             n = n.root();