curl: backport upstream fix to 7.77.0 regression

Backport upstream curl commit `ee97f1769` (schannel: set ALPN length
correctly for HTTP/2, 2021-05-26) to get a fix to curl issue 7138,
a regression in 7.77.0.

Fixes: #22355

Utilities/cmcurl/lib/vtls/schannel.c

@@ -874,6 +874,7 @@ schannel_connect_step1(struct Curl_easy *data, struct connectdata *conn,
 
 #ifdef USE_HTTP2
     if(data->state.httpwant >= CURL_HTTP_VERSION_2) {
+      alpn_buffer[cur++] = ALPN_H2_LENGTH;
       memcpy(&alpn_buffer[cur], ALPN_H2, ALPN_H2_LENGTH);
       cur += ALPN_H2_LENGTH;
       infof(data, "schannel: ALPN, offering %s\n", ALPN_H2);