add defensive asserts

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1686404 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/util/fst/BytesStore.java

@@ -98,6 +98,8 @@ class BytesStore extends DataOutput implements Accountable {
     while (len > 0) {
       int chunk = blockSize - nextWrite;
       if (len <= chunk) {
+        assert b != null;
+        assert current != null;
         System.arraycopy(b, offset, current, nextWrite, len);
         nextWrite += len;
         break;