fix commit problem in qsql_ibase.cpp

Transaction with unsuccessful commit can never be rollbacked. This migth cause some metaobjects get locked and as result hanging of any client (including itself) which tries to use that metaobjects. Example:

1. given the following:

   CREATE SEQUENCE myseq;
    
   CREATE TABLE mytbl (id INTEGER);
    
   CREATE OR ALTER TRIGGER mytrig_bi0 FOR mytbl
   active BEFORE INSERT POSITION 0
   AS
   BEGIN
     NEW.id = NEXT VALUE FOR myseq;
   END

2. qt code:

     QSqlDatabase db = ...