libmagic_port.c: use size_t for read/write sizes

src/libmagic_port.c:382:16: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]

382 |   } while (got < len);
    |            ~~~ ^ ~~~

src/libmagic_port.c:394:18: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]

394 |   } while (wrote < len);
    |            ~~~~~ ^ ~~~