bgpd: handle NULL pointers in lcommunity_cmp()

bgpd: handle NULL pointers in lcommunity_cmp()

Like community_cmp() and ecommunity_cmp(), the lcommunity_cmp() function
also needs to handle NULL pointers for correct operation.

Without this fix, bgpd can crash when entering the following commands:
vtysh -c "configure terminal" -c "ip large-community-list standard WORD deny"
vtysh -c "configure terminal" -c "no ip large-community-list expanded WORD"

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>