bgpd: fix crash when polling bgp4v2PathAttrTable

bgpd:  fix crash when polling bgp4v2PathAttrTable

we have

(gdb) p *path->attr->aspath
$1 = {refcnt = 3, segments = 0x0, json = 0x0, str = 0x55723d0b7470 "", str_len = 0, asnotation = ASNOTATION_PLAIN}

It looks like this aspath is empty, resulting in a size 0 and NULL pointer for path->attr->aspath->segments which leads to the SIGSEGV

fixe: return 0 when segments is null.

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