zebra: Handle crash when backpointer does not exist

zebra: Handle crash when backpointer does not exist

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==3058==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000010 (pc 0x7f5bf3ef7477 bp 0x7ffdfaa20d40 sp 0x7ffdfaa204c8 T0)
==3058==The signal is caused by a READ memory access.
==3058==Hint: address points to the zero page.
    #0 0x7f5bf3ef7476 in memcpy /build/glibc-OTsEL5/glibc-2.27/string/../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:134
    #1 0x4d158a in __asan_memcpy (/usr/lib/frr/zebra+0x4d158a)
    #2 0x7f5bf58da8ad in stream_put /home/qlyoung/frr/lib/stream.c:605:3
    #3 0x67d428 in zsend_ipset_entry_notify_owner /home/qlyoung/frr/zebra/zapi_msg.c:851:2
    #4 0x5c70b3 in zebra_pbr_add_ipset_entry /home/qlyoung/frr/zebra/zebra_pbr.c
    #5 0x68e1bb in zread_ipset_entry /home/qlyoung/frr/zebra/zapi_msg.c:2465:4
    #6 0x68f958 in zserv_handle_commands /home/qlyoung/frr/zebra/zapi_msg.c:2611:3
    #7 0x55666d in main /home/qlyoung/frr/zebra/main.c:309:2
    #8 0x7f5bf3e5db96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #9 0x4311d9 in _start (/usr/lib/frr/zebra+0x4311d9)

the ipset->backpointer was NULL as that the hash lookup failed to find
anything.  Prevent this crash from happening.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>