libfdt: fix undefined behaviour in fdt_splice_()

libfdt: fix undefined behaviour in fdt_splice_()

Along the lines of commit d0b3ab0a ("libfdt: Fix undefined behaviour
in fdt_offset_ptr()"), fdt_splice_() similarly may not use pointer
arithmetic to do overflow checks. (The left side of the checks added by
d4c7c25c ["libfdt: check for potential overrun in _fdt_splice()"]
doesn't really lend itself to similar replacement though.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Message-Id: <f2d09e81-7cb8-c5cc-9699-1ac05b0626ff@suse.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>