netfilter: ebtables: fix memory leak when blob is malformed

The bug fix was incomplete, it "replaced" crash with a memory leak.
The old code had an assignment to "ret" embedded into the conditional,
restore this.

Fixes: 7997eff8

 ("netfilter: ebtables: reject blobs that don't provide all entry points")
Reported-and-tested-by:  <syzbot+a24c5252f3e3ab733464@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>