If an error occurs and channel_detector_exit() is called, it relies on
entries of the 'detectors' array to be NULL.
Otherwise, it may access to un-initialized memory.
Fix it and initialize the memory, as what was done before the commit in
Fixes.
Fixes: a063b650ce5d ("ath: dfs_pattern_detector: Avoid open coded arithmetic in memory allocation")
Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>