cookies: fix NULL dereference if flushing cookies with no CookieInfo set

Regression brought by a52e46f390 (shipped in 7.63.0)

Closes #3613

lib/cookie.c

@@ -1542,6 +1542,7 @@ static int cookie_output(struct CookieInfo *c, const char *dumphere)
   struct Cookie **array;
 
   /* at first, remove expired cookies */
+  if(c)
     remove_expired(c);
 
   if(!strcmp("-", dumphere)) {
@@ -1561,7 +1562,7 @@ static int cookie_output(struct CookieInfo *c, const char *dumphere)
         "# This file was generated by libcurl! Edit at your own risk.\n\n",
         out);
 
-  if(c->numcookies) {
+  if(c && c->numcookies) {
     array = malloc(sizeof(struct Cookie *) * c->numcookies);
     if(!array) {
       if(!use_stdout)