lib/cf-h1-proxy: silence compiler warnings (gcc 14)

lib/cf-h1-proxy: silence compiler warnings (gcc 14)

They came up ealier with gcc 12 (Windows), but apparently gcc 14 is
still reporting them, also under Linux.

```
/home/runner/work/curl-for-win/curl-for-win/curl/lib/cf-h1-proxy.c: In function 'cf_h1_proxy_close':
/home/runner/work/curl-for-win/curl-for-win/curl/lib/cf-h1-proxy.c:1060:17: warning: null pointer dereference [-Wnull-dereference]
 1060 |   cf->connected = FALSE;
/home/runner/work/curl-for-win/curl-for-win/curl/lib/cf-h1-proxy.c:1061:8: warning: null pointer dereference [-Wnull-dereference]
 1061 |   if(cf->ctx) {
      |      ~~^~~~~
In function 'tunnel_free',
    inlined from 'cf_h1_proxy_destroy' at /home/runner/work/curl-for-win/curl-for-win/curl/lib/cf-h1-proxy.c:1053:3:
/home/runner/work/curl-for-win/curl-for-win/curl/lib/cf-h1-proxy.c:198:27: warning: null pointer dereference [-Wnull-dereference]
  198 |   struct h1_tunnel_state *ts = cf->ctx;
      |                           ^~
```
Ref: https://github.com/curl/curl-for-win/actions/runs/8985369476/job/24679219528#step:3:6320

Fixes #13237
Closes #13555