of/fdt: Don't clear initial_boot_params if fdt_check_header() fails

If the device tree pointer is NULL, early_init_dt_verify() fails, leaving
initial_boot_params unchanged.  If the device tree pointer is non-NULL but
invalid, early_init_dt_verify() again fails but this time it also clears
initial_boot_params.

Leave initial_boot_params unchanged if the device tree pointer is invalid.
This doesn't fix a bug, but it makes the behavior more consistent and
easier to analyze.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>