thermal: core: Fix thermal zone lookup by ID

When a thermal zone is looked up by an ID and no zone is found matching
that ID, the thermal_zone_get_by_id() function will return a pointer to
the thermal zone list head which isn't actually a valid thermal zone.

This can lead to a subsequent crash because a valid pointer is returned
to the called, but dereferencing that pointer as struct thermal_zone is
not safe.

Fixes: 329b064f

 ("thermal: core: Get thermal zone by id")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200724170105.2705467-1-thierry.reding@gmail.com