A slight change for stability

While updating ie_cgenericelement_uaf earlier today, I noticed the
changes made it a tiny bit less stable. Juan's test log in #1809
also kinda shows that (with the first attempt failing), so I decided
to go back and move the string crafting part, that way between
CollectGarbage() and the overwrite, there is less noise, and hopefully
more stable.  I did a few tests, seems better.
Land #1810, @wchen-r7's modification to make ie_cgenericelement_uaf more stable