fix segfault when trying to open a non existing device

Signed-off-by: Ludwig Nussel <ludwig.nussel@suse.de>

Index: cryptsetup-luks-1.0.4_SVN30/lib/setup.c
===================================================================
--- cryptsetup-luks-1.0.4_SVN30.orig/lib/setup.c
+++ cryptsetup-luks-1.0.4_SVN30/lib/setup.c
@@ -608,7 +608,7 @@ out:
 
 static int __crypt_luks_open(int arg, struct setup_backend *backend, struct crypt_options *options)
 {
-	struct luks_masterkey *mk;
+	struct luks_masterkey *mk=NULL;
 	struct luks_phdr hdr;
 	char *password; int passwordLen;
 	struct device_infos infos;