String::c_ptr may abort

the function String::c_ptr() has a debug assert to ensure there was room for a /0 terminator. This may be false in the following series.

Start with a string where str_length == Alloced_length - 1.
Append a single character such that this is called:

  inline bool append(char chr)

  {

    if (str_length < Alloced_length)

    {

      Ptr[str_length++]=chr;

    }

    else

    {

      if (realloc_with_extra(str_length + 1))

	return 1;

      Ptr[str_length++]=chr;

    }

    return 0;

  }

The character is added, wiping out the /0 and creating a situation where str_length == Alloced_length and if c_ptr() is called next, the debug assert aborts.