rsi: fix missing null pointer check from a failed ieee80211_probereq_get call

The call to ieee80211_probereq_get can return NULL if a skb allocation
fails, so add a null pointer check and free an earlier skb on the error
exit return path.

Fixes: 7fdcb8e1 ("rsi: add support for hardware scan offload")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>