PDFDocument::Tokenize() in the aKeyword == "obj" case allocates a
PDFObjectElement, stores it as an owning pointer inside rElements, and
also stores two non-owning references to it in m_aOffsetObjects and
m_aIDObjects. So make sure those 2 other containers are also cleared
then elements go away.

LO_TRACE="valgrind" bin/run pdfverify <sample>

doesn't report errors anymore after the fix.

Change-Id: Ie103de3e24a1080257a79e53b994e8536a9597bc
Reviewed-on: https://gerrit.libreoffice.org/50627Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>