when calculating table->null_fields.


mysql-test/r/create.result:
  Testcase for BUG#14480
mysql-test/t/create.test:
  Testcase for BUG#14480
sql/sql_table.cc:
  BUG#14480: For CREATE ... SELECT ... a field list passed to mysql_prepare_table() contains
  instances of create_field for both create-list and select-list. mysql_prepare_table() 
  matches elements that refer to the same field, and joins them together. When the "join"  
  is performed, both of create_field structures has already been counted in "null_fields". 
  
  This fix makes sure that "null_fields" contains the correct value after two create_field 
  structures have been joined.