bug: Fix possible null pointer when /First does not exist

[eknoes]

Seems to be a typo, otherwise this can lead to the following null pointer when /First does not exist.

Program received signal SIGSEGV, Segmentation fault.
0x0000555555789696 in PDFInteger::GetValue (this=0x0) at PDF-Writer/PDFWriter/PDFInteger.cpp:34
34		return mValue;

#0  0x0000555555789696 in PDFInteger::GetValue (this=0x0)
    at PDF-Writer/PDFWriter/PDFInteger.cpp:34
#1  0x000055555574de05 in PDFParser::ParseExistingInDirectStreamObject (this=0x7ffff5d09060, inObjectId=10)
    at PDF-Writer/PDFWriter/PDFParser.cpp:1762
#2  0x000055555574ad36 in PDFParser::ParseNewObject (this=0x7ffff5d09060, inObjectId=10)
    at PDF-Writer/PDFWriter/PDFParser.cpp:730
#3  0x000055555573fed6 in PDFParser::ParsePagesObjectIDs (this=0x7ffff5d09060)
    at PDF-Writer/PDFWriter/PDFParser.cpp:848
#4  0x000055555573b6a1 in PDFParser::StartPDFParsing (this=0x7ffff5d09060, inSourceStream=0x7ffff5d09020, inOptions=...)
    at PDF-Writer/PDFWriter/PDFParser.cpp:139

[galkahana]

Thank you!