Problem: ALTER TABLE ADD INDEX may lead to table copying if there's
numeric field(s) with non-default display width modificator specified.

Fix: compare numeric field's storage lenghts when we decide whether 
they can be considered 'equal' for table alteration purposes.


mysql-test/r/error_simulation.result:
  Fix for bug#50946: fast index creation still seems to copy the table
    - test result.
mysql-test/t/error_simulation.test:
  Fix for bug#50946: fast index creation still seems to copy the table
    - test case.
sql/field.cc:
  Fix for bug#50946: fast index creation still seems to copy the table
    - check numeric field's pack lengths instead of it's display lenghts
  comparing fields equality for table alteration purposes.
sql/sql_table.cc:
  Fix for bug#50946: fast index creation still seems to copy the table
    - check compare_tables() result for testing purposes.