subprocess: add a callback to ignore output

This callback is equivalent to shoving data into /dev/null.

osdep/subprocess.c

@@ -45,6 +45,10 @@ static void *run_subprocess(void *ptr)
     return NULL;
 }
 
+void mp_devnull(void *ctx, char *data, size_t size)
+{
+}
+
 void mp_subprocess_detached(struct mp_log *log, char **args)
 {
     struct subprocess_args *p = talloc_zero(NULL, struct subprocess_args);

osdep/subprocess.h

@@ -24,6 +24,8 @@ struct mp_cancel;
 
 typedef void (*subprocess_read_cb)(void *ctx, char *data, size_t size);
 
+void mp_devnull(void *ctx, char *data, size_t size);
+
 // Start a subprocess. Uses callbacks to read from stdout and stderr.
 int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
                   subprocess_read_cb on_stdout, subprocess_read_cb on_stderr,