detect: config checks alstate before getting tx

Ticket: 4972

As is done in detect-lua-extensions.
We can have a flow with alproto unknown, no state, and therefore
cannot run AppLayerParserGetTx which could try to run a NULL
function

src/detect-config.c

@@ -89,6 +89,9 @@ void DetectConfigRegister(void)
 static void ConfigApplyTx(Flow *f,
         const uint64_t tx_id, const DetectConfigData *config)
 {
+    if (f->alstate == NULL) {
+        return;
+    }
     void *tx = AppLayerParserGetTx(f->proto, f->alproto, f->alstate, tx_id);
     if (tx) {
         AppLayerTxData *txd = AppLayerParserGetTxData(f->proto, f->alproto, tx);