nl80211: check nla_nest_start() return value

Coverity pointed out that we might dereference NULL later
if nla_nest_start() returns a failure. This isn't really
true since we'd bomb out before, but we should check the
return value directly, so do that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: 7fa322c878d70e38675f50e17acdce7fa3f5ac8c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: Id72a2ea390ae7c017a87bc678b0cc280dd0b3a5f
Signed-off-by: Ahmad Kholaif <akholaif@codeaurora.org>