UDP socket resource not freed after close

If the application creates a UDP socket using lwIP Posix api function "lwip_socket()" or lwIP low level api "udp_new()" the socket resource (pcb) is allocated from a resource pool. However, when the application closes such a socket, the socket resource is not released to the pool causing a resource leak.

It is expected that the socket resource is released back to the resource pool.

The problem does not affect UDP Sockets allocated with Hilscher specific API  "LwIP_Socket_Alloc()", "LwIP_Socket_Socket()" and the Socket API exposed via DPM mailbox to  applications.