patch 8.0.0044

Problem:    In diff mode the cursor may end up below the last line, resulting
            in an ml_get error.
Solution:   Check the line to be valid.

src/testdir/test_diffmode.vim

@@ -218,3 +218,20 @@ func Test_diffoff()
   bwipe!
   bwipe!
 endfunc
+
+func Test_setting_cursor()
+  new Xtest1
+  put =range(1,90)
+  wq
+  new Xtest2
+  put =range(1,100)
+  wq
+  
+  tabe Xtest2
+  $
+  diffsp Xtest1
+  tabclose
+
+  call delete('Xtest1')
+  call delete('Xtest2')
+endfunc