UBSan runtime error: division by zero

...triggered by CppunitTest_sw_ooxmlexport.

Though it iss unclear to me what is the best fix in this case, emit a
style:legend-expansion-aspect-ratio value of 1, emit a style:legend-expansion
value of "custom" without an accompanying style:legend-expansion-aspect-ratio
(if that is even valid), emit another style:legend-expansion value, or emit no
style:legend-expansion attribute (if that is even valid)?

Change-Id: I36afe35082a841974bb2480fe11a7a3dd815ddf0