add contiguous block of rows case

Author: Marie BONBOIRE

src/sage/matrix/matrix_modn_dense_template.pxi

@@ -3017,6 +3017,10 @@ cdef class Matrix_modn_dense_template(Matrix_dense):
         if ncols < 0 or col < 0 or col + ncols > self._ncols:
             raise IndexError("columns out of range")
 
+        if col == 0 and ncols == self._ncols:
+            cdef Matrix_modn_dense_template M = self.new_matrix(nrows=nrows, ncols=self._ncols)
+            memcpy(M._entries, self._entries+row*ncols, sizeof(celement)*ncols*nrows)
+
         cdef Matrix_modn_dense_template M = self.new_matrix(nrows=nrows, ncols=ncols)
         cdef Py_ssize_t i,r
         for i,r in enumerate(range(row, row+nrows)) :