Server failed in assert() when we tried to create a DECIMAL() temp field
with a scale of more than the allowed 30. Now we limit the scale to the
allowed maximum. A truncation warning is thrown as necessary.


mysql-test/r/type_newdecimal.result:
  Show that out of range DECIMAL temp fields will no longer
  stop the server with an assert.
mysql-test/t/type_newdecimal.test:
  Show that out of range DECIMAL temp fields will no longer
  stop the server with an assert.
sql/sql_select.cc:
  When creating DECIMAL() temp field, ascertain we stay within allowed
  limits. If not, truncate and warn.