bug in PdfString utf8 constructor

[RobertWilbrandt]

Creating a PdfString from utf8 fails with multibyte characters. A minimal example is creating a pdfstring from the utf8-string "iß". My guess for the reason is the +1 in the declaration of the sourceEnd variable in the "ConvertUTF8toUTF16" function in PdfString.cpp, but i had no time to validate that.