plug a DIR* leak on error path

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/find_root_device.c b/libbb/find_root_device.c
index 32c86ce..8436cd6 100644
--- a/libbb/find_root_device.c
+++ b/libbb/find_root_device.c
@@ -29,14 +29,15 @@
 	char *retpath = NULL;
 	int len, rem;
 
-	dir = opendir(ap->devpath);
-	if (!dir)
-		return NULL;
-
 	len = strlen(ap->devpath);
 	rem = DEVNAME_MAX-2 - len;
 	if (rem <= 0)
 		return NULL;
+
+	dir = opendir(ap->devpath);
+	if (!dir)
+		return NULL;
+
 	ap->devpath[len++] = '/';
 
 	while ((entry = readdir(dir)) != NULL) {