Check for syntax errors in SANs in X509_check_host

OpenSSL's X509_get_ext_d2i is very difficult to use correctly and makes
it easy to mix up missing extension and invalid extension. This happened
in X509_check_host and caused it to use the common name fallback.

This does not have any real consequence:

First, such certificates have EXFLAG_INVALID set and would actually fail
to validate in the first place. (Ideally such a state would not exist at
all, and then this will be moot. See crbug.com/42290243.) This is
demostrated by having to update cert verification test expectations.

Second, even if it were reachable, one would need to somehow convince a
trusted CA to misissue a certificate with an unparseable SAN extension
*and* place a DNS name in the common name that it didn't intend
validate. An attacker that can do this can no doubt already cause the
CA to misissue a well-formed, wrong certificate too.

Nonetheless, it's good to handle errors, so do so.

Bug: 489032714
Change-Id: I5cef97e616b9af2e4b65a47d967a2980c46fd1ad
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/90227
Reviewed-by: Lily Chen <chlily@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: David Benjamin <davidben@google.com>