patch 8.0.0645: no error for illegal back reference in NFA engine

Problem:    The new regexp engine does not give an error for using a back
            reference where it is not allowed. (Dominique Pelle)
Solution:   Check the back reference like the old engine. (closes #1774)
diff --git a/src/testdir/test_statusline.vim b/src/testdir/test_statusline.vim
index cf85bd5..351b119 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -223,7 +223,7 @@
   set statusline=ab%(cd%q%)de
   call assert_match('^abde\s*$', s:get_statusline())
   copen
-  call assert_match('^abcd\[Quickfix List\1]de\s*$', s:get_statusline())
+  call assert_match('^abcd\[Quickfix List]de\s*$', s:get_statusline())
   cclose
 
   " %#: Set highlight group. The name must follow and then a # again.