V4L/DVB: v4l: mem2mem_testdev: fix g_fmt NULL pointer dereference

Calling g_fmt before s_fmt resulted in a NULL pointer dereference as no
default formats were being selected on probe.

Reported-by: Németh Márton <[e-mail]>
Signed-off-by: Pawel Osciak <[e-mail]>
Signed-off-by: Kyungmin Park <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/video/mem2mem_testdev.c

···
 		goto err_m2m;
 	}
 
+	q_data[V4L2_M2M_SRC].fmt = &formats[0];
+	q_data[V4L2_M2M_DST].fmt = &formats[0];
+
 	return 0;
 
 err_m2m: