String::chop() is wrong and may potentially crash.

Olivier noticed that this code looks wrong in sql_string.h:

  inline void chop()

  {

    Ptr[str_length--]= '\0';

  }

it should be written as:

  inline void chop()

  {

    Ptr[--str_length]= '\0';

  }

The reason why the problem was not found is probably because all chop() callers do not really care about correct 0-termination, they only need to reduce length by 1. Perhaps it should be fixed not to maintain 0 termination at all, to something like this:

  inline void chop()

  {

    str_length--;

  }