NetworkPkg: Fixes to static code analysis hits

Introducing fixes to memory leak issues identified by static code analysis
tool.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>

NetworkPkg/Dhcp6Dxe/Dhcp6Io.c

@@ -203,6 +203,7 @@ Dhcp6EnqueueRetry (
     //
     // Unexpected message type.
     //
+    FreePool(TxCb);
     return EFI_DEVICE_ERROR;
   }
 

NetworkPkg/Ip4Dxe/Ip4If.c

@@ -491,9 +491,13 @@ Ip4CreateInterface (
   IP4_INTERFACE             *Interface;
   EFI_SIMPLE_NETWORK_MODE   SnpMode;
 
+  if (Mnp == NULL) {
+    return NULL;
+  }
+
   Interface = AllocatePool (sizeof (IP4_INTERFACE));
 
-  if ((Interface == NULL) || (Mnp == NULL)) {
+  if (Interface == NULL) {
     return NULL;
   }
 

NetworkPkg/TcpDxe/TcpDispatcher.c

@@ -390,6 +390,7 @@ TcpAttachPcb (
                   );
   if (EFI_ERROR (Status)) {
     IpIoRemoveIp (IpIo, Tcb->IpInfo);
+    FreePool (Tcb);
     return Status;
   }