Character width sizes are wrong if the font cmap table is incomplete

This "hack" will use "0" as the character if the real character is not present in the table; as "0" is reasonably likely to exist if it also doesn't exist it will fall back to the old code of returning 0.

This should for the most part fix: https://github.com/devongovett/pdfkit/issues/169 as it is caused by the same issue of fonts not having full cmap tables.