mailbox: mailbox-test: set tdev->signal to NULL after freeing

tdev->signal is not set NULL after it's freed. This will cause random
exceptions when the stale pointer is accessed after tdev->signal is
freed. Also, since tdev->signal allocation is skipped the next time
it's written, this leads to continuous fault finally leading to the
total death of the system.

Fixes: d1c2f87c ("mailbox: mailbox-test: Prevent memory leak")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>