foreach() and list_delete() don't mix.

foreach() and list_delete() don't mix.

Fix crash when releasing duplicate entries in the encoding conversion cache
list, caused by releasing the current entry of the list being chased by
foreach().  We have a standard idiom for handling such cases, but this
loop wasn't using it.

This got broken in my recent rewrite of GUC assign hooks.  Not sure how
I missed this when testing the modified code, but I did.  Per report from
Peter.