Fix crash when canceling a partially edited transaction

[Didier Vidal]

prompt> cvs -q diff -u Transaction.c
Index: Transaction.c
===================================================================
RCS file: /home/cvs/cvsroot/gnucash/src/engine/Transaction.c,v
retrieving revision 1.261.4.15
diff -u -r1.261.4.15 Transaction.c
--- Transaction.c       1 Oct 2005 18:08:50 -0000       1.261.4.15
+++ Transaction.c       2 Oct 2005 07:23:50 -0000
@@ -1455,7 +1455,8 @@
 xaccTransBeginEdit (Transaction *trans)
 {
        if(!trans) { return; }
-    qof_begin_edit(&trans->inst);
+
+    QOF_BEGIN_EDIT(&trans->inst);
  
    if (qof_book_shutting_down(trans->inst.book))
      return;