libfdt: Fix undefined behaviour in fdt_offset_ptr()

libfdt: Fix undefined behaviour in fdt_offset_ptr()

Using pointer arithmetic to generate a pointer outside a known object is,
technically, undefined behaviour in C.  Unfortunately, we were using that
in fdt_offset_ptr() to detect overflows.

To fix this we need to do our bounds / overflow checking on the offsets
before constructing pointers from them.

Reported-by: David Binderman <[email protected]>
Signed-off-by: David Gibson <[email protected]>
Signed-off-by: Simon Glass <[email protected]>