cgi_param_multiple() is broken, potentially causing endless loop in actual usage

[kennyk-peplink]

It's typical (if not only) use case:

while ((value = cgi_param_multiple(name)) != NULL) {
// processing here...
}

will cause endless loop when name is found at the end of the CGI parameter list.

[kennyk-peplink]

6691523

Currently cgi_param_multiple() stored the "next iterator" for repeated usage.
When the last CGI parameter is match and fetched, however, we've got a value and "next iteration" is NULL;
subsequently call will redo a fetching from the start, falling into endless loop.

[kennyk-peplink]

Suggested fix: Simply store the 'iterator', not the 'next iterator'; if there is a next fetch, start from that iterator's next.

1. Replace "if (!iter) iter = formvars_start" with,
   iter = iter ? iter->next : formvars_start;

2. Remove "iter = iter->next;" before break;

[LeSpocky]

Personally I have currently no usecase for cgi_param_multiple(), but I would be happy if you could provide a patch and a testcase.