int → ptrdiff_t for read results

int → ptrdiff_t for read results

* src/fileio.c (Finsert_file_contents): Use ptrdiff_t, not int, to
store results returned by emacs_fd_read, which return ptrdiff_t.
This doesn’t fix any bugs since the values happen to fit in int,
but is a bit clearer and may simplify future changes.

(cherry picked from commit 8ac78986ff7028c249f136b3523dacf0693c22d3)