Do not access DB_CONFIG when db_home is not set

Fixes CVE-2017-10140:
https://bugzilla.redhat.com/show_bug.cgi?id=1464032#c9

Downloaded from
http://pkgs.fedoraproject.org/cgit/rpms/libdb.git/commit/?id=8047fa8580659fcae740c25e91b490539b8453eb

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

--- db-5.3.28/src/env/env_open.c.old	2017-06-26 10:32:11.011419981 +0200
+++ db-5.3.28/src/env/env_open.c	2017-06-26 10:32:46.893721233 +0200
@@ -473,7 +473,7 @@
 	env->db_mode = mode == 0 ? DB_MODE_660 : mode;
 
 	/* Read the DB_CONFIG file. */
-	if ((ret = __env_read_db_config(env)) != 0)
+	if (env->db_home != NULL && (ret = __env_read_db_config(env)) != 0)
 		return (ret);
 
 	/*