patch 8.0.0513: getting name of cleared highlight group is wrong

Problem:    Getting name of cleared highlight group is wrong. (Matt Wozniski)
Solution:   Only skip over cleared names for completion. (closes #1592)
            Also fix that a cleared group causes duplicate completions.
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 8a00f99..9ebe3f1 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -326,13 +326,16 @@
   syntax keyword Bar tar
   call assert_match('Foo', execute('syntax'))
   call assert_match('Bar', execute('syntax'))
+  call assert_equal('Foo', synIDattr(hlID("Foo"), "name"))
   syn clear Foo
   call assert_notmatch('Foo', execute('syntax'))
   call assert_match('Bar', execute('syntax'))
+  call assert_equal('Foo', synIDattr(hlID("Foo"), "name"))
   syn clear Foo Bar
   call assert_notmatch('Foo', execute('syntax'))
   call assert_notmatch('Bar', execute('syntax'))
   hi clear Foo
+  call assert_equal('Foo', synIDattr(hlID("Foo"), "name"))
   hi clear Bar
 endfunc