Fix GH-17047: UAF on iconv filter failure

The first while loop sets the bucket variable, and this is freed in
out_failure. However, when the second "goto out_failure" is triggered
th

Fix GH-17047: UAF on iconv filter failure

The first while loop sets the bucket variable, and this is freed in
out_failure. However, when the second "goto out_failure" is triggered
then bucket still refers to the bucket from the first while loop,
causing a UAF.
Fix this by separating the error paths.

Closes GH-17058.

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