Commit 9ed3f222 ("intel_th: Don't reference unassigned outputs")
fixes a NULL dereference for all masters except the last one ("256+"),
which keeps the stale pointer after the output driver had been unassigned.

Fix the off-by-one.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 9ed3f222 ("intel_th: Don't reference unassigned outputs")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>