Fixed nullpointer bug in rare circumstances with empty VCs (#33)

Change-Id: Id43e6e427ed258be03494b1e82b0d504ac5a8cc9

Changes

@@ -1,9 +1,12 @@
-0.55.8 2017-07-04
+0.55.8 2017-09-05
         - [feature] Retrieve and display pagebreaks (diewald)
         - [bugfix] Handle invalid dates in input (diewald)
         - [feature] Support availability meta data (diewald)
         - [bugfix] Do not respond with a "match all" VC if the
           VC data (e.g. JSON) is wrong (diewald)
+        - [cleanup] Improved test suite for queries/VC with wildcards
+          and regexes (margaretha, diewald)
+        - [bugfix] Ignore zero-bit bitset segments in searching (diewald)
 
 0.55.7 2017-01-24
         - [bugfix] Sorted results in unordered element distance query

src/main/java/de/ids_mannheim/korap/KrillIndex.java

@@ -1351,7 +1351,12 @@ public Result search (Krill ks) {
                  * Todo: There may be a way to know early if the bitset is emty
                  * by using LongBitSet - but this may not be as fast as I think.
                  */
-                final Bits bitset = collection.bits(atomic);
+                final FixedBitSet bitset = collection.bits(atomic);
+
+				if (bitset.nextSetBit(0) == DocIdSetIterator.NO_MORE_DOCS) {
+					continue;
+				};
+
                 final PositionsToOffset pto = new PositionsToOffset(atomic,
                         field);
 

src/test/java/de/ids_mannheim/korap/index/TestMultipleDistanceIndex.java

@@ -398,6 +398,8 @@ public void testWithKrillWithCollection () throws IOException {
                 .getResource("/queries/bugs/cosmas_wildcards_qao.jsonld")
                 .getFile());
         krill = new Krill(json);
+		assertEquals(krill.getCollection().toString(),
+					 "QueryWrapperFilter(availability:/QAO.*/)");
         kr = krill.apply(sample);
         assertEquals(4, kr.getMatches().size());
     }