stp: fix misplaced break in Ieee8021dBpduSerializer causing serialization failure

The break statement in the BPDU_CFG case of the serialize() method was
placed immediately after the dynamic cast, before any of the BPDU fields
were written to the stream. This caused the serializer to only write the
4-byte BPDU header (protocolIdentifier, protocolVersionIdentifier,
bpduType = 32 bits) while the BpduCfg chunk declares chunkLength = B(35)
(280 bits).

When fcsMode was set to 'computed', the EthernetFcsInserter would
serialize the packet to compute the FCS and detect the mismatch:
  'serialized length is incorrect: serialized=32 bit, chunk=280 bit'

The fix moves the break to after all BpduCfg fields are serialized.

Author: adamgeorge309

src/inet/linklayer/ieee8021d/common/Ieee8021dBpduSerializer.cc

@@ -25,7 +25,6 @@ void Ieee8021dBpduSerializer::serialize(MemoryOutputStream& stream, const Ptr<co
     switch (bpduBase->getBpduType()) {
         case BPDU_CFG: {
             const auto& bpdu = CHK(dynamicPtrCast<const BpduCfg>(chunk));
-            break;
             stream.writeBit(bpdu->getTcaFlag());
             stream.writeNBitsOfUint64Be(bpdu->getReserved(), 6);
             stream.writeBit(bpdu->getTcFlag());
@@ -40,6 +39,7 @@ void Ieee8021dBpduSerializer::serialize(MemoryOutputStream& stream, const Ptr<co
             stream.writeUint16Be(bpdu->getMaxAge().dbl() * 256);
             stream.writeUint16Be(bpdu->getHelloTime().dbl() * 256);
             stream.writeUint16Be(bpdu->getForwardDelay().dbl() * 256);
+            break;
         }
         case BPDU_TCN: {
             const auto& bpdu = CHK(dynamicPtrCast<const BpduTcn>(chunk));