fdt: Check if the FDT address is configured

In case the "fdt addr" command wasn't ran yet and any other "fdt"
subcommand was issued, the system crashed due to NULL pointer being
used.

This is caused by "fdt addr" command setting up a pointer to the
FDT memory location. Prior issuing "fdt addr", the pointer is NULL
so calling any other subcommands crashed the u-boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>