tidy-up: assortment of small fixes

- examples/headerapi: fix wrong cast.
- curl_ngtcp2: delete stray character from error message.
- rustls: fix inline variable declaration.
- sendf: drop redundant `int` cast.
- libtest/cli_ws_data: drop cast with mismatched signedness.

Cherry-picked from #18343

Closes #18664

lib/sendf.c

@@ -857,7 +857,7 @@ static CURLcode cr_in_rewind(struct Curl_easy *data,
     Curl_set_in_callback(data, FALSE);
     CURL_TRC_READ(data, "cr_in, rewind via set.seek_func -> %d", err);
     if(err) {
-      failf(data, "seek callback returned error %d", (int)err);
+      failf(data, "seek callback returned error %d", err);
       return CURLE_SEND_FAIL_REWIND;
     }
   }

lib/vquic/curl_ngtcp2.c

@@ -1337,7 +1337,7 @@ out:
   result = Curl_1st_err(result, cf_progress_egress(cf, data, &pktx));
   result = Curl_1st_err(result, check_and_set_expiry(cf, data, &pktx));
 
-  CURL_TRC_CF(data, cf, "[%" FMT_PRId64 "] cf_recv(blen=%zu) -> %dm, %zu",
+  CURL_TRC_CF(data, cf, "[%" FMT_PRId64 "] cf_recv(blen=%zu) -> %d, %zu",
               stream ? stream->id : -1, blen, result, *pnread);
   CF_DATA_RESTORE(cf, save);
   return result;

lib/vtls/rustls.c

@@ -1212,13 +1212,14 @@ cr_connect(struct Curl_cfilter *cf,
       }
       if(data->set.ssl.certinfo) {
         size_t num_certs = 0;
+        size_t i;
         while(rustls_connection_get_peer_certificate(rconn, (int)num_certs)) {
           num_certs++;
         }
         result = Curl_ssl_init_certinfo(data, (int)num_certs);
         if(result)
           return result;
-        for(size_t i = 0; i < num_certs; i++) {
+        for(i = 0; i < num_certs; i++) {
           const rustls_certificate *cert;
           const unsigned char *der_data;
           size_t der_len;