__ieee80211_amsdu_copy_frag intentionally initializes a pointer to
array[-1] to increment it later to valid values. clang rightfully
generates an array-bounds warning on the initialization statement.

Initialize the pointer to array[0] and change the algorithm from
increment before to increment after consume.
Signed-off-by: NMatthias Kaehlcke <mka@chromium.org>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>