patch 8.1.0458: ml_get error and crash when using "do"

Problem:    Ml_get error and crash when using "do".
Solution:   Adjust cursor position also when diffupdate is not needed.
            (Hirohito Higashi)

src/testdir/test_diffmode.vim

@@ -277,6 +277,28 @@ func Test_dp_do_buffer()
   %bwipe!
 endfunc
 
+func Test_do_lastline()
+  e! one
+  call setline(1, ['1','2','3','4','5','6'])
+  diffthis
+
+  new two
+  call setline(1, ['2','4','5'])
+  diffthis
+
+  1
+  norm dp]c
+  norm dp]c
+  wincmd w
+  call assert_equal(4, line('$'))
+  norm G
+  norm do
+  call assert_equal(3, line('$'))
+
+  windo diffoff
+  %bwipe!
+endfunc
+
 func Test_diffoff()
   enew!
   call setline(1, ['Two', 'Three'])