tests: Fix testbgpmpattr and make check, broken by BGP NHT.
* bgp_mp_attr_test.c: Fix segfault due to uninitialised bgp_nexthop_cache
globals in nexthop parsing path, which require bgp_scan_init() to have
been called.
Fixes issue introduced with:
'bgpd, zebra: Use next hop tracking for connected routes too'