Use a invalid GUID instead of 0 as default

If a packet does not have a sign/guid, use an invalid guid instead of 0
as the default, so that these packets get tracked against the invalid
guid which can be excluded while updating the port's stream stats.

common/sign.h

@@ -83,6 +83,7 @@ public:
 
     static quint32 magic();
     static bool packetGuid(const uchar *pkt, int pktLen, uint *guid);
+    static const quint32 kMaxGuid = 0x00ffffff;
 private:
     static const quint32 kSignMagic = 0x1d10c0da; // coda! (unicode - 0x1d10c)
     static const quint8 kTypeLenEnd = 0x00;