common:iobuf: Exclude cases with IOBUF_INPUT_TEMP/IOBUF_OUTPUT_TEMP.

* common/iobuf.c (iobuf_read): Handle a case with IOBUF_INPUT_TEMP.
(iobuf_write): Handle a case with IOBUF_OUTPUT_TEMP.

• GnuPG-bug-id: T5941
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>