add suggested _matrix

Author: Marie BONBOIRE

src/sage/matrix/matrix_modn_dense_template.pxi

@@ -3020,12 +3020,12 @@ cdef class Matrix_modn_dense_template(Matrix_dense):
         cdef Matrix_modn_dense_template M = self.new_matrix(nrows=nrows, ncols=ncols)
 
         if col == 0 and ncols == self._ncols:
-            memcpy(M._entries, self._entries+row*ncols, sizeof(celement)*ncols*nrows)
+            memcpy(M._entries, self._matrix[row], sizeof(celement)*ncols*nrows)
             return M
 
         cdef Py_ssize_t i,r
         for i,r in enumerate(range(row, row+nrows)) :
-            memcpy(M._entries + (i*ncols), self._entries+self._ncols*r+col, sizeof(celement)*ncols)
+            memcpy(M._matrix[i], self._matrix[r]+col, sizeof(celement)*ncols)
 
         return M