Avoid seemingly-possible overflow

By looking just at that test, it looks like 2 + dn_size could overflow. In
fact that can't happen as that would mean we've read a CA cert of size is too
big to be represented by a size_t.

However, it's best for code to be more obviously free of overflow without
having to reason about the bigger picture.

library/ssl_srv.c

@@ -2584,7 +2584,9 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
     {
         dn_size = crt->subject_raw.len;
 
-        if( end < p || (size_t)( end - p ) < 2 + dn_size )
+        if( end < p ||
+            (size_t)( end - p ) < dn_size ||
+            (size_t)( end - p ) < 2 + dn_size )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
             break;