nlmsg_parse() might return an error, so test its return value before
potential random memory accesses.

Errors introduced in commit 115c9b81 (rtnetlink: Fix problem with
buffer allocation)
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>