nstat: NULL Dereference when no entries specified

nstat: NULL Dereference when no entries specified

The NULL Pointer Dereference vulnerability happens in load_ugly_table(), misc/nstat.c, in the latest version of iproute2.
The vulnerability can be triggered by:
1. db is set to NULL at struct nstat_ent *db = NULL;
2. n is set to NULL at n = db;
3. NULL dereference of variable n happens at sscanf(p+1, "%llu", &n->val) != 1

Signed-off-by: ZiAo Li <23110240084@m.fudan.edu.cn>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>