rustls: typecast variable for safer trace output

This is a variadic function call with a mismatched argument type; on
platforms where uintptr_t and size_t differ, this invokes undefined
behavior.

Reported in Joshua's sarif data

Closes #18628

lib/vtls/rustls.c

@@ -121,7 +121,7 @@ read_cb(void *userdata, uint8_t *buf, uintptr_t len, uintptr_t *out_n)
     connssl->peer_closed = TRUE;
   *out_n = (uintptr_t)nread;
   CURL_TRC_CF(io_ctx->data, io_ctx->cf, "cf->next recv(len=%zu) -> %d, %zu",
-              len, result, nread);
+              (size_t)len, result, nread);
   return ret;
 }