ath: dfs_pattern_detector: Fix possible null-pointer dereference in channel_detector_create()

kzalloc() is used to allocate memory for cd->detectors, and if it fails,
channel_detector_exit() behind the label fail will be called:
  channel_detector_exit(dpd, cd);

In channel_detector_exit(), cd->detectors is dereferenced through:
  struct pri_detector *de = cd->detectors[i];

To fix this possible null-pointer dereference, check cd->detectors before
the for loop to dereference cd->detectors.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210805153854.154066-1-islituo@gmail.com