Fixed a bug where we could ask for a read when there were none in the pool (that's a bad thing).

git-svn-id: file:///humgen/gsa-scr1/gsa-engineering/svn_contents/trunk@605 348d0f76-0448-11de-a6fe-93d51630548a

java/src/org/broadinstitute/sting/gatk/iterators/MergingSamRecordIterator2.java

@@ -150,7 +150,11 @@ public class MergingSamRecordIterator2 implements CloseableIterator<SAMRecord>,
         if (!initialized) {
             lazyInitialization();
         }
+
         final ComparableSamRecordIterator iterator = this.pq.poll();
+        if (iterator == null) {
+            return null;
+        }
         final SAMRecord record = iterator.next();
         addIfNotEmpty(iterator);