nl80211: nl80211hdr_put() doesn't return an ERR_PTR

There are a few places which check nl80211hdr_put() for an ERR_PTR
but actually it returns NULL on error and never error values.  In
nl80211_testmode_dump() the return wasn't checked at all so I have
added one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[some whitespace changes]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: cb35fba360dfc3496e5d8a47e23ec5ccdfd90925
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[akholaif@codeaurora.org: resolved conflicts manually]
Change-Id: I5da1aafea35110a1ce606d402b1b96859d34b869
Signed-off-by: Ahmad Kholaif <akholaif@codeaurora.org>