[REVIEW-3-6] fix for fdo#54558, ext entries in extlst do not need to have a corresponding normal entry

[Markus Mohrhard]

Hey,

after studying the ooxml spec again the right fix for the other crash
with the test document from fdo#54558 is that we need to handle
missing elements for the corresponding extLst. The ooxml spec allows
to have the ext entry but not the corresponding normal element.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=fcd85be5d5e44b00c6fa5717c7ec13ea657483d4