Comment 7 for bug 1571584

Also seen in R2.21.x build 37:

(gdb) bt
#0 0x00007f16963c2cc9 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1 0x00007f16963c60d8 in __GI_abort () at abort.c:89
#2 0x00007f16963bbb86 in __assert_fail_base (fmt=0x7f169650c830 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x110c0db "state_count_[listener] == 0",
    file=file@entry=0x110c0b4 "controller/src/db/db_table.cc", line=line@entry=89,
    function=function@entry=0x110c480 <DBTableBase::ListenerInfo::Unregister(int)::__PRETTY_FUNCTION__> "void DBTableBase::ListenerInfo::Unregister(DBTableBase::ListenerId)")
    at assert.c:92
#3 0x00007f16963bbc32 in __GI___assert_fail (assertion=0x110c0db "state_count_[listener] == 0", file=0x110c0b4 "controller/src/db/db_table.cc", line=89,
    function=0x110c480 <DBTableBase::ListenerInfo::Unregister(int)::__PRETTY_FUNCTION__> "void DBTableBase::ListenerInfo::Unregister(DBTableBase::ListenerId)") at assert.c:101
#4 0x0000000000f193ef in Unregister (listener=2, this=0x7f168406cb00) at controller/src/db/db_table.cc:89
#5 DBTableBase::Unregister (this=0x7f168406c990, listener=2) at controller/src/db/db_table.cc:182
#6 0x0000000000b2553e in RouteFlowUpdate::WalkDone (partition=<optimized out>, info=0x7f16840a7880) at controller/src/vnsw/agent/pkt/flow_table.cc:2121
#7 0x0000000000f1e27b in operator() (a0=<optimized out>, this=<optimized out>) at /usr/include/boost/function/function_template.hpp:767
#8 DBTableWalker::Worker::Run (this=0x7f1641ee2e20) at controller/src/db/db_table_walker.cc:151
#9 0x000000000102334c in TaskImpl::execute (this=0x7f168fc23c40) at controller/src/base/task.cc:253
#10 0x00007f1696f91b3a in ?? () from /usr/lib/libtbb.so.2
#11 0x00007f1696f8d816 in ?? () from /usr/lib/libtbb.so.2
#12 0x00007f1696f8cf4b in ?? () from /usr/lib/libtbb.so.2
#13 0x00007f1696f890ff in ?? () from /usr/lib/libtbb.so.2
#14 0x00007f1696f892f9 in ?? () from /usr/lib/libtbb.so.2
#15 0x00007f16971ad182 in start_thread (arg=0x7f168f247700) at pthread_create.c:312
#16 0x00007f169648647d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111