writerfilter: fix ubsan complaints about "-42" enum value

Apparently the StyleType enum is specific to the domain-mapper, the
OOXML filter has integer constants in its model.xml file...

Also surprisingly the section styles only exist in RTF; w_ST_StyleType
in "ECMA-376 3rd Edition" only contains the 4 values of StyleType enum.

Change-Id: I5407800f801824676df309b3705e233cf1382721