x86: fix disassembler build after 1a3b4f90bc5f

x86: fix disassembler build after 1a3b4f90bc5f

In commit 1a3b4f90bc5f ("x86: convert two pointers to (indexing)
integers") I neglected the fact that compilers may warn about comparing
ptrdiff_t (signed long) with size_t (unsigned long) values. Since just
before we've checked that the value is positive, simply add a cast
(despite my dislike for casts).