Abort on unrecognised warning alerts

A peer continually sending unrecognised warning alerts could mean that we
make no progress on a connection. We should abort rather than continuing if
we receive an unrecognised warning alert.

Thanks to Shi Lei for reporting this issue.
Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/record/rec_layer_s3.c

@@ -1351,9 +1351,15 @@ int ssl3_read_bytes(SSL *s, int type, int *recvd_type, unsigned char *buf,
                 goto f_err;
             }
 #ifdef SSL_AD_MISSING_SRP_USERNAME
-            else if (alert_descr == SSL_AD_MISSING_SRP_USERNAME)
-                return (0);
+            else if (alert_descr == SSL_AD_MISSING_SRP_USERNAME) {
+                return 0;
+            }
 #endif
+            else {
+                al = SSL_AD_HANDSHAKE_FAILURE;
+                SSLerr(SSL_F_SSL3_READ_BYTES, SSL_R_UNKNOWN_ALERT_TYPE);
+                goto f_err;
+            }
         } else if (alert_level == SSL3_AL_FATAL) {
             char tmp[16];