...of this-ptr of in-construction FormFieldSignature while still in the base
FormField ctor, as happens (in the xpdfimport process) during
CppunitTest_xmlsecurity_signing:

> Form.cc:448:12: runtime error: downcast of address 0x60f000000040 which does not point to an object of type 'FormFieldSignature'
> 0x60f000000040: note: object is of type 'FormField'
>  03 00 00 6d  50 84 f2 00 00 00 00 00  03 00 00 00 04 00 00 00  00 00 00 00 01 be be be  07 00 00 00
>               ^~~~~~~~~~~~~~~~~~~~~~~
>               vptr for 'FormField'
>     #0 0x73d7f4 in FormWidgetSignature::FormWidgetSignature(PDFDoc*, Object*, unsigned int, Ref, FormField*) workdir/UnpackedTarball/poppler/poppler/Form.cc:448:12
>     #1 0x741713 in FormField::_createWidget(Object*, Ref) workdir/UnpackedTarball/poppler/poppler/Form.cc:677:34
>     #2 0x73e747 in FormField::FormField(PDFDoc*, Object*, Ref const&, FormField*, std::set<int, std::less<int>, std::allocator<int> >*, FormFieldType) workdir/UnpackedTarball/poppler/poppler/Form.cc:547:7
>     #3 0x74ec2b in FormFieldSignature::FormFieldSignature(PDFDoc*, Object*, Ref const&, FormField*, std::set<int, std::less<int>, std::allocator<int> >*) workdir/UnpackedTarball/poppler/poppler/Form.cc:1383:5
>     #4 0x740d7f in Form::createFieldFromDict(Object*, PDFDoc*, Ref const&, FormField*, std::set<int, std::less<int>, std::allocator<int> >*) workdir/UnpackedTarball/poppler/poppler/Form.cc:1700:19
>     #5 0x750727 in Form::Form(PDFDoc*, Object*) workdir/UnpackedTarball/poppler/poppler/Form.cc:1623:33
>     #6 0x71d440 in Catalog::getForm() workdir/UnpackedTarball/poppler/poppler/Catalog.cc:1042:18
>     #7 0x708572 in Annots::createAnnot(Dict*, Object*) workdir/UnpackedTarball/poppler/poppler/Annot.cc:7218:41
>     #8 0x70772b in Annots::Annots(PDFDoc*, int, Object*) workdir/UnpackedTarball/poppler/poppler/Annot.cc:7128:17
>     #9 0x87f3e9 in Page::getAnnots(XRef*) workdir/UnpackedTarball/poppler/poppler/Page.cc:405:18
>     #10 0x8814b0 in Page::displaySlice(OutputDev*, double, double, int, bool, bool, int, int, int, int, bool, bool (*)(void*), void*, bool (*)(Annot*, void*), void*, bool) workdir/UnpackedTarball/poppler/poppler/Page.cc:611:15
>     #11 0x880ff6 in Page::display(OutputDev*, double, double, int, bool, bool, bool, bool (*)(void*), void*, bool (*)(Annot*, void*), void*, bool) workdir/UnpackedTarball/poppler/poppler/Page.cc:521:3
>     #12 0x88d906 in PDFDoc::displayPage(OutputDev*, int, double, double, int, bool, bool, bool, bool (*)(void*), void*, bool (*)(Annot*, void*), void*, bool) workdir/UnpackedTarball/poppler/poppler/PDFDoc.cc:491:20
>     #13 0x5d59fb in main sdext/source/pdfimport/xpdfwrapper/wrapper_gpl.cxx:142:14
>     #14 0x7f11600ea730 in __libc_start_main (/lib64/libc.so.6+0x20730)
>     #15 0x459388 in _start (instdir/program/xpdfimport+0x459388)

Change-Id: Ia808919c8d2363d616feb4664f314a77b40dfbb8