Parse HelloVerifyRequest: avoid buffer overread at the start

In ssl_parse_hello_verify_request, we read 3 bytes (version and cookie
length) without checking that there are that many bytes left in
ssl->in_msg. This could potentially read from memory outside of the
ssl->receive buffer (which would be a remotely exploitable
crash).

library/ssl_cli.c

@@ -1577,6 +1577,19 @@ static int ssl_parse_hello_verify_request( mbedtls_ssl_context *ssl )
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> parse hello verify request" ) );
 
+    /* Check that there is enough room for:
+     * - 2 bytes of version
+     * - 1 byte of cookie_len
+     */
+    if( mbedtls_ssl_hs_hdr_len( ssl ) + 3 > ssl->in_msglen )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1,
+            ( "incoming HelloVerifyRequest message is too short" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                    MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
+
     /*
      * struct {
      *   ProtocolVersion server_version;