Crash in response_expired()

[ivan4th]

The following crash has been observed when running e2e tests:

INFO[0827] VPP: /src/vpp/src/plugins/upf/upf_pfcp_server.h:240 (pfcp_msg_pool_elt_at_index) assertion `m->is_valid_pool_item' fails  pid=5954 what=stdout
INFO[0827] VPP:                                          pid=5954 what=stdout
INFO[0827] VPP: Program received signal SIGABRT, Aborted.  pid=5954 what=stdout
INFO[0827] VPP: 0x00007ffff4717fb7 in raise () from /lib/x86_64-linux-gnu/libc.so.6  pid=5954 what=stdout
INFO[0827] VPP: #0  0x00007ffff4717fb7 in raise () from /lib/x86_64-linux-gnu/libc.so.6  pid=5954 what=stdout
INFO[0827] VPP: #1  0x00007ffff4719921 in abort () from /lib/x86_64-linux-gnu/libc.so.6  pid=5954 what=stdout
INFO[0827] VPP: #2  0x0000000000407193 in os_panic () at /src/vpp/src/vpp/vnet/main.c:371  pid=5954 what=stdout
INFO[0827] VPP: #3  0x00007ffff55f8619 in debugger () at /src/vpp/src/vppinfra/error.c:84  pid=5954 what=stdout
INFO[0827] VPP: #4  0x00007ffff55f8397 in _clib_error (how_to_die=2, function_name=0x0, line_number=0, fmt=0x7fffac0ba8b7 "%s:%d (%s) assertion `%s' fails") at /src/vpp/src/vppinfra/error.c:143  pid=5954 what=stdout
INFO[0827] VPP: #5  0x00007fffac063138 in pfcp_msg_pool_elt_at_index (psm=0x7fffac2f7250 <pfcp_server_main>, index=11880) at /src/vpp/src/plugins/upf/upf_pfcp_server.h:240  pid=5954 what=stdout
INFO[0827] VPP: #6  0x00007fffac062052 in response_expired (id=11880) at /src/vpp/src/plugins/upf/upf_pfcp_server.c:462  pid=5954 what=stdout
INFO[0827] VPP: #7  0x00007fffac05352a in pfcp_process (vm=0x7ffff6483b80 <vlib_global_main>, rt=0x7fffb4cb3580, f=0x0) at /src/vpp/src/plugins/upf/upf_pfcp_server.c:1250  pid=5954 what=stdout
INFO[0827] VPP: #8  0x00007ffff61c9eed in vlib_process_bootstrap (_a=140736242026504) at /src/vpp/src/vlib/main.c:1454  pid=5954 what=stdout
INFO[0827] VPP: #9  0x00007ffff561c7a4 in clib_calljmp () at /src/vpp/src/vppinfra/longjmp.S:123  pid=5954 what=stdout