Fix secret_driver compile warning, bug.

Set def to NULL in secretLoad(), otherwise we can access a
random pointer on error.
Signed-off-by: NChris Lalancette <clalance@redhat.com>

src/secret/secret_driver.c

@@ -432,7 +432,7 @@ static virSecretEntryPtr
 secretLoad(virConnectPtr conn, virSecretDriverStatePtr driver,
            const char *xml_basename)
 {
-    virSecretDefPtr def;
+    virSecretDefPtr def = NULL;
     virSecretEntryPtr secret = NULL, ret = NULL;
     char *xml_filename;