thermal: Exynos: Fix NULL pointer dereference in exynos_unregister_thermal()

exynos_unregister_thermal() is functional only when 'th_zone' is not
NULL (ensured by the NULL checks). However, in the event it is NULL, it
gets dereferenced in the for loop. This patch fixes this issue.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>