Replace `sprintf` with `snprintf`.

* graph/x11/grx11.c (main), graph/xtest.c (main): Do it.

* src/ftsdf.c (BUFSIZE): New macro.
(write_header): Use `snprintf`.

* src/ftstring.c (write_header): Ditto.

* src/rsvg-port.c (rsvg_port_preset_slot): Ditto.