patch 8.2.0270: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5649)
diff --git a/src/testdir/test_join.vim b/src/testdir/test_join.vim
index 377edab..fdd9e05 100644
--- a/src/testdir/test_join.vim
+++ b/src/testdir/test_join.vim
@@ -436,5 +436,11 @@
   call setline(1, ['a', 'b', '', 'c', 'd'])
   normal 5J
   call assert_equal('a b c d', getline(1))
+  call setline(1, ['a', 'b', 'c'])
+  2,2join
+  call assert_equal(['a', 'b', 'c'], getline(1, '$'))
+  call assert_equal(2, line('.'))
+  2join
+  call assert_equal(['a', 'b c'], getline(1, '$'))
   bwipe!
 endfunc