CCK decimal: non-significant zeros when previewing

this was introduced since comment_driven unstable6 by #739962: CCK decimal: non-significant zeros conflict
when previewing decimal fields the assertion "double vs string" arises
attached patch should fix this,
nevertheless the root of the problem is in CCK #461536-7: $form_state['values'] not reflecting an accurate decimal value (i.e. non-significant zeros are dropped) and we are having all these troubles because of their misbehavior, having this fixed on their side would even let old versions of comment_driven work properly