schannel: fix `-Warith-conversion` gcc 13 warning

```
lib/vtls/schannel.c:1201:22: warning: conversion to 'unsigned int' from 'int' may change the sign of the result [-Warith-conversion]
 1201 |     *extension_len = *list_len +
      |                      ^
```

Closes #12616

diff --git a/lib/vtls/schannel.c b/lib/vtls/schannel.c
index 455269a3c4af28baa491161a360ddfb89f0c9e73..45c3373713c9a60f8d7f2798687dd6c55f7f70c9 100644 (file)
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -1198,9 +1198,8 @@ schannel_connect_step1(struct Curl_cfilter *cf, struct Curl_easy *data)
     cur += proto.len;
 
     *list_len = curlx_uitous(cur - list_start_index);
-    *extension_len = *list_len +
-      (unsigned short)sizeof(unsigned int) +
-      (unsigned short)sizeof(unsigned short);
+    *extension_len = (unsigned int)(*list_len +
+      sizeof(unsigned int) + sizeof(unsigned short));
 
     InitSecBuffer(&inbuf, SECBUFFER_APPLICATION_PROTOCOLS, alpn_buffer, cur);
     InitSecBufferDesc(&inbuf_desc, &inbuf, 1);