detect/parse: fix crash on 'internal' keyword use

When keyword __flowvar__postmatch__, an internal keyword, is used
in a rule the 'Setup' func ptr will be NULL. This caused a crash.

src/detect-parse.c

@@ -694,7 +694,7 @@ static int SigParseOptions(DetectEngineCtx *de_ctx, Signature *s, char *optstr,
 
     /* Call option parsing */
     st = SigTableGet(optname);
-    if (st == NULL) {
+    if (st == NULL || st->Setup == NULL) {
         SCLogError(SC_ERR_RULE_KEYWORD_UNKNOWN, "unknown rule keyword '%s'.", optname);
         goto error;
     }