curses ui: add missing iconv_close calls

The iconv_t are opened but never closed.

Spotted by Coverity: CID 1399708
Spotted by Coverity: CID 1399709
Spotted by Coverity: CID 1399713

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20190314172524.9290-1-samuel.thibault@ens-lyon.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>