[VPP-566] VPP crash in clear trace after packet-generator enable

[vvalderrv]

vpp crashes if the test/test_mpls.py is executed using e.g. pypy instead of cpython, in which case the "clear trace" in 'MPLS V4 Explicit NULL test' is issued very quickly following "packet-generator enable"

Program terminated with signal SIGABRT, Aborted.
#0 0x00007f33b2df0428 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
54 ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
[Current thread is 1 (Thread 0x7f33b578c740 (LWP 32563))]
(gdb) bt
#0 0x00007f33b2df0428 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
#1 0x00007f33b2df202a in __GI_abort () at abort.c:89
#2 0x00000000004089b7 in os_panic () at /home/ksekera/vpp/build-data/../vpp/vnet/main.c:330
#3 0x00007f33b3a17fad in debugger () at /home/ksekera/vpp/build-data/../vppinfra/vppinfra/error.c:84
#4 0x00007f33b3a183b5 in _clib_error (how_to_die=2, function_name=0x0, line_number=0, fmt=0x7f33b4cf50b8 "%s:%d (%s) assertion `%s' fails")
at /home/ksekera/vpp/build-data/../vppinfra/vppinfra/error.c:143
#5 0x00007f33b4ce9498 in vlib_validate_trace (tm=0x68d3f8 <vlib_global_main+664>, b=0x7f3371dbdf00)
at /home/ksekera/vpp/build-data/../vlib/vlib/trace_funcs.h:51
#6 0x00007f33b4ce94fb in vlib_add_trace (vm=0x68d160 <vlib_global_main>, r=0x7f3372dd6d40, b=0x7f3371dbdf00, n_data_bytes=32)
at /home/ksekera/vpp/build-data/../vlib/vlib/trace_funcs.h:62
#7 0x00007f33b4ce9855 in vlib_trace_frame_buffers_only (vm=0x68d160 <vlib_global_main>, node=0x7f3372dd6d40, buffers=0x7f337431cd10, n_buffers=256,
next_buffer_stride=4, n_buffer_data_bytes_in_trace=32) at /home/ksekera/vpp/build-data/../vlib/vlib/trace.c:75
#8 0x00007f33b43cce12 in ethernet_input_inline (vm=0x68d160 <vlib_global_main>, node=0x7f3372dd6d40, from_frame=0x7f337431cd00,
variant=ETHERNET_INPUT_VARIANT_ETHERNET) at /home/ksekera/vpp/build-data/../vnet/vnet/ethernet/node.c:307
#9 0x00007f33b43ce213 in ethernet_input (vm=0x68d160 <vlib_global_main>, node=0x7f3372dd6d40, from_frame=0x7f337431cd00)
at /home/ksekera/vpp/build-data/../vnet/vnet/ethernet/node.c:733
#10 0x00007f33b4c9a9d9 in dispatch_node (vm=0x68d160 <vlib_global_main>, node=0x7f3372dd6d40, type=VLIB_NODE_TYPE_INTERNAL,
dispatch_state=VLIB_NODE_STATE_POLLING, frame=0x7f337431cd00, last_time_stamp=9634362601290608) at /home/ksekera/vpp/build-data/../vlib/vlib/main.c:996
#11 0x00007f33b4c9b055 in dispatch_pending_node (vm=0x68d160 <vlib_global_main>, p=0x7f33734ec8b8, last_time_stamp=9634362601290608)
at /home/ksekera/vpp/build-data/../vlib/vlib/main.c:1134
#12 0x00007f33b4c9ce85 in vlib_main_loop (vm=0x68d160 <vlib_global_main>) at /home/ksekera/vpp/build-data/../vlib/vlib/main.c:1543
#13 0x00007f33b4c9d4f6 in vlib_main (vm=0x68d160 <vlib_global_main>, input=0x7f3372fbbfb0) at /home/ksekera/vpp/build-data/../vlib/vlib/main.c:1678
#14 0x00007f33b4f2055d in thread0 (arg=6869344) at /home/ksekera/vpp/build-data/../vlib/vlib/unix/main.c:485
#15 0x00007f33b3a3b3b0 in clib_calljmp () at /home/ksekera/vpp/build-data/../vppinfra/vppinfra/longjmp.S:110
#16 0x00007ffe34972490 in ?? ()
#17 0x00007f33b4f209a7 in vlib_unix_main (argc=13, argv=0x7ffe34973718) at /home/ksekera/vpp/build-data/../vlib/vlib/unix/main.c:545
#18 0x00000000004086a4 in main (argc=13, argv=0x7ffe34973718) at /home/ksekera/vpp/build-data/../vpp/vnet/main.c:271