Plug memory leak in xaccSplitDestroy

Splits were not marked for deletion if the transaction is read-only
and the account is not marked for deletion yet. The net result is
that split will not be freed later on.
However xaccSplitDestroy is also called from a Transaction's do_destroy.
At that point accounts are not necessarily marked for deletion yet (like
is the case when a datafile is closed). This turned out to be a problem
for invoice post transactions (which are also read only) and hence
would cause memory to leak.

libgnucash/engine/Split.c

@@ -1447,6 +1447,7 @@ xaccSplitDestroy (Split *split)
     acc = split->acc;
     trans = split->parent;
     if (acc && !qof_instance_get_destroying(acc)
+        && !qof_instance_get_destroying(trans)
         && xaccTransGetReadOnly(trans))
         return FALSE;