r21740: this fixes the real cause of the large log files we had. The problem

r21740: this fixes the real cause of the large log files we had. The problem
was we were not checking the result of a convert_string() call, and it
was giving -1. We then passed -1 to fwrite() on stdout, which on aix
and macosx wrote all of available memory to stdout :)

To fix this, replace non-printing chars with ? in d_printf if the
string cannot be converted