media: nxp: imx8-isi: Check whether crossbar pad is non-NULL before access

In the Linux kernel, the following vulnerability has been resolved: media: nxp: imx8-isi: Check whether crossbar pad is non-NULL before access When translating source to sink streams in the crossbar subdev, the driver tries to locate the remote subdev connected to the sink pad. The remote pad may be NULL, if userspace tries to enable a stream that ends at an unconnected crossbar sink. When that occurs, the driver dereferences the NULL pad, leading to a crash. Prevent the crash by checking if the pad is NULL before using it, and return an error if it is.