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
6 jobs for pdfdoc_setup_isok in 20 minutes and 49 seconds (queued for 1 minute and 53 seconds)
Status Job ID Name Coverage
  Build
passed #71580
build

00:04:05

passed #71585
build_android

00:01:06

passed #71581
build_clang_libcpp

00:03:54

passed #71584
build_clazy

00:04:06

passed #71582
build_gcc_49

00:08:15

passed #71583
build_mingw64_fedora

00:15:33