PDFDoc::setup: Fix return value

At that point xref can have gone wrong since extractPDFSubtype() can
have caused a reconstruct that broke stuff so instead of unconditionally
returning true, return xref->isOk()

Fixes #706
diff --git a/poppler/PDFDoc.cc b/poppler/PDFDoc.cc
index 351bd2e..9213f56 100644
--- a/poppler/PDFDoc.cc
+++ b/poppler/PDFDoc.cc
@@ -311,7 +311,7 @@
   extractPDFSubtype();
 
   // done
-  return true;
+  return xref->isOk();
 }
 
 PDFDoc::~PDFDoc() {