[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.

The parser now also rejects chunk sizes and Content-Length values that would overflow ssize_t, so they get a malformed request error instead of wrapping.

Fixes GH-24536