media: st-hva: Fix potential NULL pointer dereferences

When ctx_id >= HVA_MAX_INSTANCES in hva_hw_its_irq_thread() it tries to
access fields of ctx that is NULL at that point. The patch gets rid of
these accesses.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>