g10: Push compress filter only if compressed.

* g10/compress.c (handle_compressed): Fix memory leak.

(backport from STABLE-BRANCH-2-2 commit:
c31abf84659dbda5503dd9f3aa3449520bcd1b84)

All other calls of push_compress_filter checks ALGO,
so, do it here, too.

• GnuPG-bug-id: T3898
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>