pdfwrite - Guard against trying to output an infinite number

pdfwrite - Guard against trying to output an infinite number

Bug #699255 " Buffer overflow on pprintg1 due to mishandle postscript file data to pdf"

The file uses an enormous parameter to xyxhow, causing an overflow in
the calculation of text positioning (value > 1e39).

Since this is basically a nonsense value, and PostScript only supports
real values up to 1e38, this patch follows the same approach as for
a degenerate CTM, and treats it as 0.

Adobe Acrobat Distiller throws a limitcheck error, so we could do that
instead if this approach proves to be a problem.