cfg80211: fix NULL pointer derference when querying regdb

Some drivers may call this function when regdb is not initialized yet,
so we need to make sure regdb is valid before trying to access it.

Make sure regdb is initialized before trying to access it in
reg_query_regdb_wmm() and query_regdb().

Reported-by: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>