media: stm32-csi: Fix dereference before NULL check

In the Linux kernel, the following vulnerability has been resolved: media: stm32-csi: Fix dereference before NULL check In 'stm32_csi_start', 'csidev->s_subdev' is dereferenced directly while assigning a value to the 'src_pad'. However the same value is being checked against NULL at a later point of time indicating that there are chances that the value can be NULL. Move the dereference after the NULL check.