patch 8.0.0269: may get ml_get error when :perldo deletes lines

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

src/testdir/test_perl.vim

@@ -82,6 +82,21 @@ function Test_perldo()
   1
   call assert_false(search('\Cperl'))
   bw!
+
+  " Check deleting lines does not trigger ml_get error.
+  new
+  call setline(1, ['one', 'two', 'three'])
+  perldo VIM::DoCommand("%d_")
+  bwipe!
+
+  " Check switching to another buffer does not trigger ml_get error.
+  new
+  let wincount = winnr('$')
+  call setline(1, ['one', 'two', 'three'])
+  perldo VIM::DoCommand("new")
+  call assert_equal(wincount + 1, winnr('$'))
+  bwipe!
+  bwipe!
 endfunc
 
 function Test_VIM_package()