of: Add check to of_scan_flat_dt() before accessing initial_boot_params

commit 3ec75441 upstream.

An empty __dtb_start to __dtb_end section might result in
initial_boot_params being null for arch/mips/ralink. This showed that the
boot process hangs indefinitely in of_scan_flat_dt().

Signed-off-by: Tobias Wolf <dev-NTEO@vplace.de>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14605/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>