mac80211: fix possible oob and null pointer dereferenced issues

Add check to avoid array supp_rates out of bound access in function
sta_apply_parameters() and also add skb null pointer check before
access.

Change-Id: Ieeb2868b10019b5748732e28688ede36ca72b0cc
Signed-off-by: Huashan Qu <hqu@codeaurora.org>