base64_decode: fix potential keyword parsing issue (CID 1340064)

src/detect-base64-decode.c

@@ -310,6 +310,9 @@ static int DetectBase64DecodeSetup(DetectEngineCtx *de_ctx, Signature *s,
         }
         else {
             sm_list = SigMatchListSMBelongsTo(s, pm);
+            if (sm_list < 0) {
+                goto error;
+            }
         }
     }