patch 8.0.0265: may get ml_get error when :pydo deletes lines

Problem:    May get ml_get error when :pydo deletes lines or switches to
            another buffer. (Nikolai Pavlov, issue #1421)
Solution:   Check the buffer and line every time.

src/testdir/test_python2.vim

@@ -0,0 +1,24 @@
+" Test for python 2 commands.
+" TODO: move tests from test87.in here.
+
+if !has('python')
+  finish
+endif
+
+func Test_pydo()
+  " Check deleting lines does not trigger ml_get error.
+  py import vim
+  new
+  call setline(1, ['one', 'two', 'three'])
+  pydo vim.command("%d_")
+  bwipe!
+
+  " Check switching to another buffer does not trigger ml_get error.
+  new
+  let wincount = winnr('$')
+  call setline(1, ['one', 'two', 'three'])
+  pydo vim.command("new")
+  call assert_equal(wincount + 1, winnr('$'))
+  bwipe!
+  bwipe!
+endfunc