Since ea890b1d "tdf#108408: support unit
specifications for ST_HpsMeasure", the OOXMLUniversalMeasureValue ctor is
converting textual data to mnValue via intermediary double instead of sal_Int32,
so textual data representing negative values now triggers UBSan warnings (e.g.,
"writerfilter/source/ooxml/OOXMLPropertySet.cxx:630:43: runtime error: -70 is
outside the range of representable values of type 'unsigned int'" during
CppunitTest_chart2_export; it appears that, while HpsMeasure may be documented
to only cover positive values, TwipsMeasure may be negative).

But OOXMLUniversalMeasureValue::mnValue is apparently only used in
OOXMLUniversalMeasureValue::getInt, to return an int value, so just change its
type.

Change-Id: I44eabb78f09100c05cc9d1e79a739648f34ea743