PKCS#7: fix unitialized boolean 'want'

The boolean want is not initialized and hence garbage. The default should
be false (later it is only set to true on tne sinfo->authattrs check).

Found with static analysis using CoverityScan

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Howells <dhowells@redhat.com>