efi/fdt: fix panic when no valid fdt found

In the Linux kernel, the following vulnerability has been resolved:

efi/fdt: fix panic when no valid fdt found

setup_arch() would invoke efi_init()->efi_get_fdt_params(). If no

valid fdt found then initial_boot_params will be null. So we

should stop further fdt processing here. I encountered this

issue on risc-v.