Fixed bug #66830 (Empty header causes PHP built-in web server to hang).

We had an infinite loop in sapi_cli_server_send_headers(): while iterating over
the linked list of headers, when an empty header was hit, continue would go to
the next iteration of the loop without updating h to be the next value in the
linked list. Updating it to always increment regardless of whether the header
is actually valid or not fixes the issue.