Previously, ngx_utf8_decode() was called from ngx_utf8_length() with
incorrect length, potentially resulting in out-of-bounds read when
handling invalid UTF-8 strings.

In practice out-of-bounds reads are not possible though, as autoindex, the
only user of ngx_utf8_length(), provides null-terminated strings, and
ngx_utf8_decode() anyway returns an errors when it sees a null in the
middle of an UTF-8 sequence.

Reported by Yunbin Liu.

If OCSP stapling was enabled with dynamic certificate loading, with some
OpenSSL versions (1.0.2o and older, 1.1.0h and older; fixed in 1.0.2p,
1.1.0i, 1.1.1) a segmentation fault might happen.

The reason is that during an abbreviated handshake the certificate
callback is not called, but the certificate status callback was called
(https://github.com/openssl/openssl/issues/1662), leading to NULL being
returned from SSL_get_certificate().

Fix is to explicitly check SSL_get_certificate() result.