coverity/56795: Fix NULL dereference in :syn keyword non-printable

Bug was introduced 3 years earlier, in 13848aa: NULL keyword_copy was 
incorrectly treated as an indicator of OOM.

test/functional/ex_cmds/syntax_spec.lua

@@ -0,0 +1,17 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local clear = helpers.clear
+local exc_exec = helpers.exc_exec
+
+describe(':syntax', function()
+  before_each(clear)
+
+  describe('keyword', function()
+    it('does not crash when group name contains unprintable characters',
+    function()
+      eq('Vim(syntax):E669: Unprintable character in group name',
+         exc_exec('syntax keyword \024 foo bar'))
+    end)
+  end)
+end)