Don't assume ToUnicode will always point to a CMap
Don't assume ToUnicode will always point to a CMap

It *should* always point to a CMap, but there are some PDFs in the wild
that put the wrong thing in there.

If it's not a CMap we can't use it, so ignore the value

Closes #252

yob/pdf-reader,yob/pdf-reader,yob/pdf-reader