Fix null matrix printf bug.

src/Matrix.cpp

@@ -425,10 +425,12 @@ namespace Aurora {
         return operatorMxA(&vdPowI, times, std::forward<Matrix&&>(matrix));
     }    
 
-
-
-
     void Matrix::printf() {
+        if(isNull())
+        {
+            ::printf("null matrix\n");
+            return;
+        }
         int k_count = getDimSize(2);
         int j_count = getDimSize(1);
         int complexstep = 1;