ipv4: stop exception dump when nexthop group shrinks

In the Linux kernel, the following vulnerability has been resolved:

ipv4: stop exception dump when nexthop group shrinks

Commit 4ce5dc9316de ("inet: switch inet_dump_fib() to RCU
protection") allowed IPv4 FIB dumps to run concurrently with nexthop
replacement.

fib_dump_info_fnhe() uses fib_info_num_path() as the loop bound,
then fib_info_nhc() independently reloads nh->nh_grp. If
RTM_NEWNEXTHOP replaces a group with fewer paths between those loads,
fib_info_nhc() returns NULL and the dump dereferences it through
nhc_flags.

Stop the walk when the indexed path is absent. Nexthop groups are
dense, so the current snapshot has no later path.