common: Fix put_membuf.
* common/membuf.c (put_membuf): Allow NULL for the second arg.
Cherry-picked from master commit of:
f271c6916469c0054c143adb4cee0588866a2a61
There has been such a use case in keybox-blob.c.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>