NIFI-2865: Fixed bug in StreamDemarcator that is exposed when the final bit of data in a stream is smaller than the previous and the previous demarcation ended on a buffer length boundary

This closes #1110.

Signed-off-by: Bryan Bende <bbende@apache.org>

nifi-commons/nifi-utils/src/main/java/org/apache/nifi/stream/io/util/StreamDemarcator.java

@@ -145,6 +145,7 @@ public class StreamDemarcator implements Closeable {
                 System.arraycopy(this.buffer, this.mark, this.buffer, 0, length);
                 this.index = length;
                 this.mark = 0;
+                this.readAheadLength = length;
             }
         }