lib: fix __fdt_parse_region()

If fdt_getprop() returns NULL, this indicates an error. In this case lenp
is set to an error code. But even if lenp = 0 we should not continue.

If fdt_getprop() returns a wider value than we expect this is a separate
error condition.

In both cases the device-tree is invalid.

Addresses-Coverity-ID: 1529703 ("Dereference after null check")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>