cron: don't close crontab_fd twice

[Theo Buehler]

The load_user() function gets a file descriptor from process_crontab().
It fdopen()s it directly and fclose()s the resulting stream. Then
process_crontab() closes the stream a second time before exiting.

Since crontab_fd is not load_user()'s descriptor, let's dup it before
opening the stream.

Index: usr.sbin/cron/user.c
===================================================================
RCS file: /var/cvs/src/usr.sbin/cron/user.c,v
retrieving revision 1.20
diff -u -p -r1.20 user.c
--- usr.sbin/cron/user.c        7 Jun 2017 23:36:43 -0000       1.20
+++ usr.sbin/cron/user.c        18 Jan 2018 23:14:20 -0000
@@ -28,6 +28,7 @@
 #include <string.h>
 #include <syslog.h>
 #include <time.h>              /* for structs.h */
+#include <unistd.h>
 
 #include "macros.h"
 #include "structs.h"
@@ -64,11 +65,16 @@ load_user(int crontab_fd, struct passwd     
        FILE *file;
        user *u;
        entry *e;
-       int status, save_errno;
+       int fd, status, save_errno;
        char **envp = NULL, **tenvp;
 
-       if (!(file = fdopen(crontab_fd, "r"))) {
+       if ((fd = dup(crontab_fd)) == -1) {
+               syslog(LOG_ERR, "(%s) DUP (%m)", name);
+               return (NULL);
+       }
+       if (!(file = fdopen(fd, "r"))) {
                syslog(LOG_ERR, "(%s) FDOPEN (%m)", name);
+               close(fd);
                return (NULL);
        }
        CrontabFilename = name;