Try to be more consistent about the alerts we send

We are quite inconsistent about which alerts get sent. Specifically, these
alerts should be used (normally) in the following circumstances:

SSL_AD_DECODE_ERROR = The peer sent a syntactically incorrect message
SSL_AD_ILLEGAL_PARAMETER = The peer sent a message which was syntactically
correct, but a parameter given is invalid for the context
SSL_AD_HANDSHAKE_FAILURE = The peer's messages were syntactically and
semantically correct, but the parameters provided were unacceptable to us
(e.g. because we do not support the requested parameters)
SSL_AD_INTERNAL_ERROR = We messed up (e.g. malloc failure)

The standards themselves aren't always consistent but I think the above
represents the best interpretation.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3480)
diff --git a/ssl/ssl_rsa.c b/ssl/ssl_rsa.c
index 6f1c380..1ee8056 100644
--- a/ssl/ssl_rsa.c
+++ b/ssl/ssl_rsa.c
@@ -775,7 +775,7 @@
         int retval = serverinfo_find_extension(serverinfo, serverinfo_length,
                                                ext_type, out, outlen);
         if (retval == -1) {
-            *al = SSL_AD_DECODE_ERROR;
+            *al = SSL_AD_INTERNAL_ERROR;
             return -1;          /* Error */
         }
         if (retval == 0)