Fix locking in UDPEndPoint::SendMsgImpl() (#10533)

#### Problem

If `GetPCB()` were to fail, `SendMsgImpl()` would return with the
LwIP lock held.

#### Change overview

Unlock before returning.

#### Testing

None. We don't currently have a way to either (a) force `GetPCB` to
fail, or (b) detect whether the LwIP lock is held.

src/inet/UDPEndPoint.cpp

@@ -224,7 +224,6 @@ CHIP_ERROR UDPEndPoint::ListenImpl()
 
 CHIP_ERROR UDPEndPoint::SendMsgImpl(const IPPacketInfo * pktInfo, System::PacketBufferHandle && msg, uint16_t sendFlags)
 {
- CHIP_ERROR res = CHIP_NO_ERROR;
  const IPAddress & destAddr = pktInfo->DestAddress;
 
  if (!msg.HasSoleOwnership())
@@ -242,8 +241,12 @@ CHIP_ERROR UDPEndPoint::SendMsgImpl(const IPPacketInfo * pktInfo, System::Packet
  LOCK_TCPIP_CORE();
 
  // Make sure we have the appropriate type of PCB based on the destination address.
- res = GetPCB(destAddr.Type());
- ReturnErrorOnFailure(res);
+ CHIP_ERROR res = GetPCB(destAddr.Type());
+ if (res != CHIP_NO_ERROR)
+ {
+ UNLOCK_TCPIP_CORE();
+ return res;
+ }
 
  // Send the message to the specified address/port.
  // If an outbound interface has been specified, call a specific version of the UDP sendto()