Fix pesky 'Non-digit found where a digit was expected' errors

Copying new data into the DIS buffer didn't NULL-terminate the
string (because it's not actually the end of the string, so why
would it?).  The problem comes if the new data exceeds the buffer
size: it uses snprintf with "%s%s" to copy the old and new data
into the new buffer.  Since the old data isn't NULL-terminated,
you will get bogus data in the middle of your new buffer.
Merge pull request #136 from mattaezell/dis_nondigit

Fix pesky 'Non-digit found where a digit was expected' errors