zebra: Check for netlink message next fault

zebra: Check for netlink message next fault

NLMSG_NEXT decrements the buffer length (status) by
the header msg length (nlmsg_len) everytime its called.
If nlmsg_len isn't accurate and set to be larger than
what it should represent, it will cause status to
decrement passed 0. This makes NLMSG_NEXT return a
pointer that references an inaccessible address.
When that is passed to NLMSG_OK, it segfaults.

Add a check to verify that there is still something to read
before we try to.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>