session flag SNAT_SESSION_FLAG_LOAD_BALANCING is not set correctly and cause crash during NAT tests in master branch

#0 0x00007f75b7e9e428 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
#1 0x00007f75b7ea002a in __GI_abort () at abort.c:89
#2 0x00000000004074b6 in os_panic () at /home/ayourtch/vpp/build-data/../src/vpp/vnet/main.c:310
#3 0x00007f75b8684a5f in debugger () at /home/ayourtch/vpp/build-data/../src/vppinfra/error.c:84
#4 0x00007f75b8684e67 in _clib_error (how_to_die=2, function_name=0x0, line_number=0,
fmt=0x7f75753c5f80 "%s:%d (%s) assertion `%s' fails") at /home/ayourtch/vpp/build-data/../src/vppinfra/error.c:143
#5 0x00007f757530dbf7 in snat_free_outside_address_and_port (addresses=0x7f757821ba64, thread_index=0, k=0x7f7578032a80,
address_index=0) at /home/ayourtch/vpp/build-data/../src/plugins/nat/nat.c:1951
#6 0x00007f75752ff5ff in nat_free_session_data (sm=0x7f75755eec80 <snat_main>, s=0x7f7578227504, thread_index=0)
at /home/ayourtch/vpp/build-data/../src/plugins/nat/nat.c:214
#7 0x00007f757530870f in nat44_add_del_lb_static_mapping (e_addr=..., e_port=80, proto=SNAT_PROTOCOL_TCP, vrf_id=0, locals=0x0,
is_add=0 '\000', twice_nat=1 '\001', out2in_only=0 '\000', tag=0x7f7578529610 "")
at /home/ayourtch/vpp/build-data/../src/plugins/nat/nat.c:1398
#8 0x00007f7575322483 in vl_api_nat44_add_del_lb_static_mapping_t_handler (mp=0x3016464c)
at /home/ayourtch/vpp/build-data/../src/plugins/nat/nat_api.c:1276
#9 0x00007f75b9db6a2d in vl_msg_api_handler_with_vm_node (am=0x7f75b9ff3480 <api_main>, the_msg=0x3016464c,
vm=0x7f75b9da8600 <vlib_global_main>, node=0x7f757802a000) at /home/ayourtch/vpp/build-data/../src/vlibapi/api_shared.c:508
#10 0x00007f75b9dc0c0e in void_mem_api_handle_msg_i (am=0x7f75b9ff3480 <api_main>, vm=0x7f75b9da8600 <vlib_global_main>,
node=0x7f757802a000, q=0x3004d080) at /home/ayourtch/vpp/build-data/../src/vlibmemory/memory_api.c:672
#11 0x00007f75b9dc0c7d in vl_mem_api_handle_msg_main (vm=0x7f75b9da8600 <vlib_global_main>, node=0x7f757802a000)
at /home/ayourtch/vpp/build-data/../src/vlibmemory/memory_api.c:682
#12 0x00007f75b9ddb08e in vl_api_clnt_process (vm=0x7f75b9da8600 <vlib_global_main>, node=0x7f757802a000, f=0x0)
at /home/ayourtch/vpp/build-data/../src/vlibmemory/vlib_api.c:353
#13 0x00007f75b9aff8cc in vlib_process_bootstrap (_a=140142500203648) at /home/ayourtch/vpp/build-data/../src/vlib/main.c:1231
#14 0x00007f75b86a8f34 in clib_calljmp () at /home/ayourtch/vpp/build-data/../src/vppinfra/longjmp.S:110
#15 0x00007f7577f14c50 in ?? ()
#16 0x00007f75b9aff9f7 in vlib_process_startup (vm=0xac9241c75ecd4600, p=0xffffffffffffffff, f=0x7f757802a000)
at /home/ayourtch/vpp/build-data/../src/vlib/main.c:1253