CLEANUP: ssl: remove opendir call in ssl_sock_load_cert

Since commit 3180f7b55434 ("MINOR: ssl: load certificates in
alphabetical order"), `readdir` was replaced by `scandir`. We can indeed
replace it with a check on the previous `stat` call.

This micro cleanup can be a good benefit when you have hundreds of bind
lines which open TLS certificates directories in terms of syscall,
especially in a case of frequent reloads.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 124b2c6..ff8af23 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -4249,7 +4249,6 @@
 {
 	struct dirent **de_list;
 	int i, n;
-	DIR *dir;
 	struct stat buf;
 	char *end;
 	char fp[MAXPATHLEN+1];
@@ -4265,8 +4264,7 @@
 	}
 
 	if (stat(path, &buf) == 0) {
-		dir = opendir(path);
-		if (!dir) {
+		if (S_ISDIR(buf.st_mode) == 0) {
 			ckchs =  ckchs_load_cert_file(path, 0,  err);
 			if (!ckchs)
 				return ERR_ALERT | ERR_FATAL;
@@ -4355,7 +4353,6 @@
 			}
 			free(de_list);
 		}
-		closedir(dir);
 		return cfgerr;
 	}