remove initialization of unsigned_flag for Item_decimal


mysql-test/r/case.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/metadata.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_2myisam.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_3innodb.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_4heap.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_5merge.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_6bdb.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/ps_7ndb.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/type_float.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix
mysql-test/r/type_newdecimal.result:
  Bug#17048 CREATE TABLE ... SELECT truncate values
  result fix