a bit of cleanup

Author: Baunsgaard

src/main/java/org/apache/sysds/runtime/matrix/data/MatrixBlock.java

@@ -1417,13 +1417,10 @@ else if( !sparse && denseBlock!=null )  //DENSE
 	 * @return the number of non zeros
 	 */
 	public long recomputeNonZeros(int k) {
-		if(k <= 1)
-			return recomputeNonZeros();
-		if(sparse && sparseBlock!=null)
+		// fallback to single thread if k <= 1, small matrix, or sparse. 
+		if(k <= 1 || ((long) rlen * clen < 10000) || (sparse && sparseBlock!=null))
 			return recomputeNonZeros();
 		else if(!sparse && denseBlock!=null){
-			if((long) rlen * clen < 10000)
-				return recomputeNonZeros();
 			final ExecutorService pool = CommonThreadPool.get(k);
 			try {
 				List<Future<Long>> f = new ArrayList<>();