nl80211: fix compiler warning

John reported the following warning:

net/wireless/nl80211.c: In function ‘nl80211_tx_mgmt’:
net/wireless/nl80211.c:5286:8: warning: ‘hdr’ may be used uninitialized in this function

Evidently, his version of gcc isn't able to see that
when "msg" is initialized, "hdr" must also be. My
gcc, 4.6.1, can actually see that and doesn't warn.
Simply initialize the variable to NULL. That means
if the compiler was ever right we'll crash though so
isn't really optimal since it may hide warnings from
the compiler when somebody modifies this code in the
future.

Reported-by: John Linville <linville@tuxdriver.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>