Fix unix_closedir_job incorrectly checking return value of job

LWT_UNIX_CHECK_JOB was incorrectly checking the directory stream
instead of the return value of closedir.

CHANGES

@@ -21,6 +21,7 @@
   * Lwt.pick and Lwt.choose select preferentially failed promises as per
   documentation (#856, #874, Raman Varabets)
   * Use the WSA_FLAG_NO_HANDLE_INHERIT on Windows when creating sockets with WSASocket if the cloexec (non-inheritable) parameter is true. Fixes a Windows problem where a child process would inherit a supposedly non-inheritable socket. (#910, Antonin Décimo)
+  * Fix Lwt_unix.closedir incorrectly checking the return value of closedir(3). (#942, Antonin Décimo)
 
 ====== Deprecations ======
 

src/unix/unix_c/unix_closedir_job.c

@@ -30,7 +30,7 @@ static void worker_closedir(struct job_closedir *job)
 
 static value result_closedir(struct job_closedir *job)
 {
-    LWT_UNIX_CHECK_JOB(job, job->dir < 0, "closedir");
+    LWT_UNIX_CHECK_JOB(job, job->result < 0, "closedir");
     lwt_unix_free_job(&job->job);
     return Val_unit;
 }