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/diff.c

@@ -2848,18 +2848,19 @@ ex_diffgetput(exarg_T *eap)
 theend:
     diff_busy = FALSE;
     if (diff_need_update)
-    {
-	diff_need_update = FALSE;
 	ex_diffupdate(NULL);
-    }
+
+    // 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
     {
-	// 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();
-
 	// Also need to redraw the other buffers.
 	diff_redraw(FALSE);
 	apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, FALSE, curbuf);

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'])

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    458,
 /**/
     457,
 /**/