select: avoid a NULL deref in cwfds_add_sock

curl_multi_waitfds(m, NULL, ...);

=> Curl_waitfds_init(&cwfds, ufds, size);

=> Curl_waitfds_add_ps(&cwfds);

=>   cwfds_add_sock(cwfds, ...);

Would then try to use the ->wfds array while set to NULL previously.
This should not happen, which this is now also protected with an assert
to trigger debug builds if it happens.

Caught by CodeSonar

Assisted-by: Jay Satiro

Closes curl#15881

lib/select.c

@@ -503,7 +503,10 @@ static unsigned int cwfds_add_sock(struct curl_waitfds *cwfds,
                                    curl_socket_t sock, short events)
 {
   int i;
-
+  if(!cwfds->wfds) {
+    DEBUGASSERT(!cwfds->count && !cwfds->n);
+    return 1;
+  }
   if(cwfds->n <= INT_MAX) {
     for(i = (int)cwfds->n - 1; i >= 0; --i) {
       if(sock == cwfds->wfds[i].fd) {