nl80211: fix memory leak in nl80211_get_ftm_responder_stats

In nl80211_get_ftm_responder_stats, a new skb is created via nlmsg_new
named msg. If nl80211hdr_put() fails, then msg should be released. The
return statement should be replace by goto to error handling code.

Fixes: 81e54d08d9d8 ("cfg80211: support FTM responder configuration/statistics")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Link: https://lore.kernel.org/r/20191004194220.19412-1-navid.emamdoost@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
