Fix [-Wincompatible-pointer-types]

[remicollet]

On Fedora 40, with GCC 14 this is now a build error

From PHP 7.4 UPGRADING.INTERNALS

  y. The read and write operations of php_stream_ops now return ssize_t, with
     negative values indicating an error.

[dktapps]

Yup, I ran into this with clang recently too (#39)

[dktapps]

Thanks!