hw/riscv/boot: Check the error of fdt_pack()

hw/riscv/boot: Check the error of fdt_pack()

Coverity reports that we don't check the error result of fdt_pack(), so
let's save the result and assert that it is 0.

Fixes: Coverity CID 1458136
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 07325315b49d5555269f76094e4bc5296e0643b9.1626303527.git.alistair.francis@wdc.com