If the txn is null, don't derefence it

git-svn-id: file:///svn/tokudb@473 c7de825b-a66e-492c-adef-691d508d4ae1

src/ydb.c

@@ -287,7 +287,7 @@ struct __toku_dbc_internal {
 };
 			  
 int __toku_c_get (DBC *c, DBT *key, DBT *data, u_int32_t flag) {
-    int r = brt_cursor_get(c->i->c, key, data, flag, c->i->db, c->i->txn->i->tokutxn);
+    int r = brt_cursor_get(c->i->c, key, data, flag, c->i->db, c->i->txn ? c->i->txn->i->tokutxn :0);
     return r;
 }