ssl_write_handshake_msg(): Allow alert on client-side SSLv3

In SSLv3, the client sends a NoCertificate alert in response to
a CertificateRequest if it doesn't have a CRT. This previously
lead to failure in ssl_write_handshake_msg() which only accepted
handshake or CCS records.

library/ssl_tls.c

@@ -3146,11 +3146,19 @@ int mbedtls_ssl_write_handshake_msg( mbedtls_ssl_context *ssl )
     /*
      * Sanity checks
      */
-    if( ssl->out_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE &&
+    if( ssl->out_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE          &&
         ssl->out_msgtype != MBEDTLS_SSL_MSG_CHANGE_CIPHER_SPEC )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+        /* In SSLv3, the client might send a NoCertificate alert. */
+#if defined(MBEDTLS_SSL_PROTO_SSL3) && defined(MBEDTLS_SSL_CLI_C)
+        if( ! ( ssl->minor_ver      == MBEDTLS_SSL_MINOR_VERSION_0 &&
+                ssl->out_msgtype    == MBEDTLS_SSL_MSG_ALERT       &&
+                ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT ) )
+#endif /* MBEDTLS_SSL_PROTO_SSL3 && MBEDTLS_SSL_SRV_C */
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
+            return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+        }
     }
 
     if( ssl->out_msgtype == MBEDTLS_SSL_MSG_HANDSHAKE &&