ovs provided in openvswitch-2.6 version is crashed by the assertion at cmap_replace()

Issue

• ovs provided in openvswitch-2.6 version is crashed by the assertion at cmap_replace().

  A sample backtrace is

  (gdb) bt
  #0  0x00007fccd56181d7 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
  #1  0x00007fccd56198c8 in __GI_abort () at abort.c:90
  #2  0x00007fccd73f8b0e in ovs_abort_valist (err_no=err_no@entry=0, format=format@entry=0x7fccd74b5b30 "%s: assertion %s failed in %s()", args=args@entry=0x7fcca27f9a10)
      at lib/util.c:335
  #3  0x00007fccd73ff9d0 in vlog_abort_valist (module_=, message=0x7fccd74b5b30 "%s: assertion %s failed in %s()", args=args@entry=0x7fcca27f9a10) at lib/vlog.c:1204
  #4  0x00007fccd73ffa64 in vlog_abort (module=module@entry=0x7fccd77a9d00 , message=message@entry=0x7fccd74b5b30 "%s: assertion %s failed in %s()") at lib/vlog.c:1218
  #5  0x00007fccd73f88cc in ovs_assert_failure (where=where@entry=0x7fccd749af37 "lib/cmap.c:850", function=function@entry=0x7fccd749af50 <__func__.6509> "cmap_replace", 
      condition=condition@entry=0x7fccd749af34 "ok") at lib/util.c:72
  #6  0x00007fccd73549b6 in cmap_replace (cmap=cmap@entry=0x7fccd98bb070, old_node=old_node@entry=0x7fcbe036af60, new_node=new_node@entry=0x0, hash=4260861212) at lib/cmap.c:850
  #7  0x00007fccd73306da in cmap_remove (hash=, node=0x7fcbe036af60, cmap=0x7fccd98bb070) at lib/cmap.h:276
  #8  ukey_delete (ukey=0x7fcbe036af60, umap=0x7fccd98bb040) at ofproto/ofproto-dpif-upcall.c:1740
  #9  revalidator_sweep__ (revalidator=revalidator@entry=0x7fccd98e5f10, purge=purge@entry=false) at ofproto/ofproto-dpif-upcall.c:2294
  #10 0x00007fccd7331fbd in revalidator_sweep (revalidator=0x7fccd98e5f10) at ofproto/ofproto-dpif-upcall.c:2318
  #11 udpif_revalidator (arg=0x7fccd98e5f10) at ofproto/ofproto-dpif-upcall.c:885
  #12 0x00007fccd73cb3c6 in ovsthread_wrapper (aux_=) at lib/ovs-thread.c:342
  #13 0x00007fccd630cdc5 in start_thread (arg=0x7fcca27fc700) at pthread_create.c:308
  #14 0x00007fccd56da76d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113