(index-reader) Correctly handle negative offset values

When wordOffset(...) returns a negative value, it means the word isn't present in the index, and we should abort.

code/index/index-reverse/java/nu/marginalia/index/PrioReverseIndexReader.java

@@ -81,6 +81,9 @@ public class PrioReverseIndexReader {
 
         long offset = wordOffset(termId);
 
+        if (offset < 0) // No documents
+            return 0;
+
         ByteBuffer buffer = ByteBuffer.allocate(4);
         try {
             documentsChannel.read(buffer, offset);