Fix (harmless) stack-use-after-return

...as reported by -fsanitize=address CppunitTest_hwpfilter_test_hwpfilter, where
stack-local HStream stream from HwpReader::filter is still referenced from
HStreamIODev in

  HStreamIODev::close
  HStreamIODev::~HStreamIODev
  HWPFile::~HWPFile
  HwpReader::~HwpReader

which is only harmless because HStream::closeInput is a nop.

Change-Id: Idbc5539ab5f463ec6d5d72f428ce60327ebdb063
HStream::closeInput does nothing

Change-Id: I0d57e227be45bb2c098969ab24eedab30514313f
Check exit code of spawned xpdfimport

Change-Id: I4bea1ebe5c9915ad5e3a4f8fecb516bc056d060d
Improve test code

(never call CPPUNIT_ASSERT etc., which work by throwing exceptions, from a dtor)

Change-Id: I293d54eb40c2ad9205d485ccff0ffd2161257142