Simple optimzations done while reviewing code


client/mysqltest.c:
  Added options --enable-ps-warnings and --disable-ps-warnings
  (to fix failing test case)
mysql-test/t/row.test:
  Disable warnings that comes from 'parse' parth
sql/field.cc:
  Removed calls to is_null() in field functions.
  (Not needed as NULL handling is done on the level above fields)
  Indentation fixes
  Removed calls to alloca() as buffer needed was quite small.
sql/field.h:
  Indentation changes and comment fixes
sql/filesort.cc:
  Simple optimization during code review
sql/item.cc:
  Indentation fixes
  Removed some unnecessary tests (added DBUG_ASSERTS() instead)
sql/item_buff.cc:
  Indentation fixes
sql/my_decimal.cc:
  Indentation fixes
  Simple optimization
  Fixed compiler warning
sql/sql_update.cc:
  Removed unnessessary assignment