Improve pg_check_dir's handling of closedir() failures.

Avoid losing errno if readdir() fails and closedir() works.  This also
avoids leaking the directory handle when readdir() fails.  Commit
6f03927fce038096f53ca67eeab9adb24938f8a6 introduced logic to better
handle readdir() and closedir() failures, bu it missed these cases.

Extracted from a larger patch by Marco Nenciarini.

src/port/pgcheckdir.c

@@ -31,6 +31,7 @@ pg_check_dir(const char *dir)
 	int			result = 1;
 	DIR		   *chkdir;
 	struct dirent *file;
+	int			readdir_errno;
 
 	chkdir = opendir(dir);
 
@@ -58,8 +59,15 @@ pg_check_dir(const char *dir)
 		errno = 0;
 #endif
 
-	if (errno || closedir(chkdir))
+	if (errno)
 		result = -1;			/* some kind of I/O error? */
 
+	/* Close chkdir and avoid overwriting the readdir errno on success */
+	readdir_errno = errno;
+	if (closedir(chkdir))
+		result = -1;			/* error executing closedir */
+	else
+		errno = readdir_errno;
+
 	return result;
 }