Fix a memory bug of Faust created from different mex shared lib than the main...

Fix a memory bug of Faust created from different mex shared lib than the main ones (mexFaustReal/Cplx/Float).

The bug occurred for butterfly and hierarchical but other wrappers might be touched.

Bug example:
matlab -nojvm -nodisplay -Dgdb -r 'setup_FAUST;import matfaust.Faust; import matfaust.fact.butterfly; import matfaust.wht; H = full(wht(32)); F = butterfly(H); F-H'