common: Avoid undefined behavior of left shift operator.

* common/iobuf.c (block_filter): Handle an error earlier.
Make sure it's unsigned.

• GnuPG-bug-id: T4975
• Suggested-by: lutianxiong <lutianxiong@huawei.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>