media: ti-vpe: cal: Fix a NULL pointer dereference in cal_ctx_v4l2_init_formats()

In cal_ctx_v4l2_init_formats(), devm_kzalloc() is assigned to
ctx->active_fmt and there is a dereference of it after that, which could
lead to NULL pointer dereference on failure of devm_kzalloc().

Fix this bug by adding a NULL check of ctx->active_fmt.

This bug was found by a static analyzer.

Builds with 'make allyesconfig' show no new warnings, and our static
analyzer no longer warns about this code.

Fixes: 71681550 ("media: ti-vpe: cal: Move format handling to cal.c and expose helpers")
Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>