OffsetCurveSetBuilder::addRingSide(): fix memleak
OffsetCurveSetBuilder::addRingSide(): fix memleak

Spotted by valgrind on test_geos_unit

mwtoews/libgeos,mwtoews/libgeos,mwtoews/libgeos,mwtoews/libgeos,mwtoews/libgeos,mwtoews/libgeos,mwtoews/libgeos