replaced get_field(MEM_ROOT *mem, Field *field) with 
get_field(MEM_ROOT *mem, Field *field, String *res).
It allows to avoid strlen().


mysql-test/r/information_schema.result:
  Fix for bug#18177 any access to INFORMATION_SCHEMA.ROUTINES crashes
  test case
mysql-test/t/information_schema.test:
  Fix for bug#18177 any access to INFORMATION_SCHEMA.ROUTINES crashes
  test case