vim-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)

df77cef92e

src/nvim/diff.c

@@ -2739,15 +2739,19 @@ void ex_diffgetput(exarg_T *eap)
 theend:
   diff_busy = false;
   if (diff_need_update) {
-    diff_need_update = false;
     ex_diffupdate(NULL);
-  } else {
+  }
+
   // Check that the cursor is on a valid character and update it's
   // position.  When there were filler lines the topline has become
   // invalid.
   check_cursor();
   changed_line_abv_curs();
 
+  if (diff_need_update) {
+    // redraw already done by ex_diffupdate()
+    diff_need_update = false;
+  } else {
     // Also need to redraw the other buffers.
     diff_redraw(false);
     apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, false, curbuf);

src/nvim/testdir/test_diffmode.vim

@@ -275,6 +275,28 @@ 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'])