[php-src] PR #24543: Fix GH-24536: Built-in server crashes on a huge chunk size or Content-Length

Pull Request: https://github.com/php/php-src/pull/24543 Author: lazerg The first on_body call pre-allocated parser->content_length bytes, which is whatever size the client declared. A chunk size of FFFFFFFFFFFFFFFF wraps to -1 and becomes a SIZE_MAX allocation, but a valid huge size like 7FFFFFFFFFFFFFFF or Content-Length: 9000000000000000000 kills the server the same way. The buffer was shrunk by the perealloc() right after anyway, so I dropped the pre-allocation and let perealloc() grow the buffer from NULL. Fixes GH-24536