Fix alt chains bug

This is a follow up to the alternate chains certificate forgery issue
(CVE-2015-1793). That issue is exacerbated in 1.0.1 by a related bug which
means that we *always* check for an alternative chain, even if we have
already found a chain. The code is supposed to stop as soon as it has found
one (and does do in master and 1.0.2).

Reviewed-by: Stephen Henson <steve@openssl.org>