common: Use iobuf_get_noeof to avoid undefined behaviors.

* common/iobuf.c (block_filter): Use iobuf_get_noeof.

Cherry-pick from master commit of:
f80346f42df4bdc7d0a9741c3922129aceae4f81

When singed integer has negative value, left shift computation is
undefined in C.

• GnuPG-bug-id: T4093
• Reported-by: Philippe Antoine
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>