Fix a minor warning about ignoring the result of write(2).

--- src/mon.c	2013-12-01 18:34:08.000000000 +0100
+++ src/mon.c	2015-09-09 22:42:21.241394637 +0200
@@ -112,7 +112,8 @@
   snprintf(buf, 32, "%d", pid);
   int fd = open(file, O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
   if (fd < 0) perror("open()");
-  write(fd, buf, 32);
+  ssize_t wrote = write(fd, buf, 32);
+  if (wrote != 32) perror("write()");
   close(fd);
 }
 
