Fix to avoid possible NPE

Author: idodeclare

opengrok-indexer/src/main/java/org/opengrok/indexer/web/SearchHelper.java

@@ -468,7 +468,7 @@ private void maybeRedirectToMatchOffset(int docID, List<String> contextFields)
         Query rewritten = query.rewrite(reader);
         Weight weight = rewritten.createWeight(searcher, ScoreMode.COMPLETE_NO_SCORES, 1);
         Matches matches = weight.matches(leaf, docID);
-        if (matches != MatchesUtils.MATCH_WITH_NO_TERMS) {
+        if (matches != null && matches != MatchesUtils.MATCH_WITH_NO_TERMS) {
             int matchCount = 0;
             int offset = -1;
             for (String field : contextFields) {