bgpd: fix crash in *bgpv2PeerErrorsTable

bgpd: fix crash in *bgpv2PeerErrorsTable

following crash occurs:
    at ./nptl/pthread_kill.c:44
    at ./nptl/pthread_kill.c:78
    at ./nptl/pthread_kill.c:89
    context=0x7ffd06d3d300)
    at /build/make-pkg/output/_packages/cp-routing/src/lib/sigevent.c:246
    length=0x7ffd06d3da88, exact=1, var_len=0x7ffd06d3da90, write_method=<optimized out>)
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_snmp_bgp4v2.c:364
    vp=vp@entry=0x7f7c88b584c0 <bgpv2_variables>, vp_len=vp_len@entry=102,
    ename=ename@entry=0x7f7c88b58440 <bgpv2_trap_oid>, enamelen=enamelen@entry=8,
    name=name@entry=0x7f7c88b58480 <bgpv2_oid>, namelen=namelen@entry=7,
    iname=0x7ffd06d3e7b0, index_len=1, trapobj=0x7f7c88b53b80 <bgpv2TrapBackListv6>,
    trapobjlen=6, sptrap=2 '\002')
    at /build/make-pkg/output/_packages/cp-routing/src/lib/agentx.c:382
    vp_len=vp_len@entry=102, ename=ename@entry=0x7f7c88b58440 <bgpv2_trap_oid>,
    enamelen=enamelen@entry=8, name=name@entry=0x7f7c88b58480 <bgpv2_oid>,
    namelen=namelen@entry=7, iname=0x7ffd06d3ec30, inamelen=16,
    trapobj=0x7f7c88b53b80 <bgpv2TrapBackListv6>, trapobjlen=6, sptrap=2 '\002')
    at /build/make-pkg/output/_packages/cp-routing/src/lib/agentx.c:298
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_snmp_bgp4v2.c:1496
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_fsm.c:48
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_fsm.c:1314
    event=Receive_NOTIFICATION_message)
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_fsm.c:2665
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_packet.c:3129
    at /build/make-pkg/output/_packages/cp-routing/src/lib/event.c:1979
    at /build/make-pkg/output/_packages/cp-routing/src/lib/libfrr.c:1213
    at /build/make-pkg/output/_packages/cp-routing/src/bgpd/bgp_main.c:510

it's due to function bgpv2PeerErrorsTable returning
return SNMP_STRING(msg_str);
with msg_str NULL rather the string ""

this commit avoid the issue.

Signed-off-by: Francois Dumontet <francois.dumontet@6wind.com>