panics when compressing empty data

[Teddy Heinen]

Hi! Attempting to compress empty data (my example uses compress_seekable but I have tested a file also) causes a panic.

fn main() {
    let cursor = std::io::Cursor::new(&[]);
    let mut out = Vec::new();
    zopfli::compress_seekable(
        &zopfli::Options::default(),
        &zopfli::Format::Gzip,
        cursor,
        &mut out,
    );
}

thread 'main' panicked at 'attempt to subtract with overflow', /home/sky/git/zopfli/src/deflate.rs:311:19
stack backtrace:
   0: rust_begin_unwind
             at /rustc/45e2c2881d11324d610815bfff097e25c412199e/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/45e2c2881d11324d610815bfff097e25c412199e/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/45e2c2881d11324d610815bfff097e25c412199e/library/core/src/panicking.rs:48:5
   3: zopfli::deflate::calculate_block_symbol_size_small
             at /home/sky/git/zopfli/src/deflate.rs:311:19
   4: zopfli::deflate::calculate_block_symbol_size_given_counts
             at /home/sky/git/zopfli/src/deflate.rs:345:9
   5: zopfli::deflate::try_optimize_huffman_for_rle
             at /home/sky/git/zopfli/src/deflate.rs:856:20
   6: zopfli::deflate::get_dynamic_lengths
             at /home/sky/git/zopfli/src/deflate.rs:905:5
   7: zopfli::deflate::calculate_block_size
             at /home/sky/git/zopfli/src/deflate.rs:836:31
   8: zopfli::squeeze::lz77_optimal
             at /home/sky/git/zopfli/src/squeeze.rs:506:20
   9: zopfli::deflate::blocksplit_attempt
             at /home/sky/git/zopfli/src/deflate.rs:1147:17
  10: zopfli::deflate::deflate_part
             at /home/sky/git/zopfli/src/deflate.rs:164:31
  11: zopfli::deflate::deflate
             at /home/sky/git/zopfli/src/deflate.rs:104:9
  12: zopfli::gzip::gzip_compress
             at /home/sky/git/zopfli/src/gzip.rs:49:5
  13: zopfli::compress
             at /home/sky/git/zopfli/src/lib.rs:98:25
  14: zopfli::compress_seekable
             at /home/sky/git/zopfli/src/lib.rs:83:5
  15: scratch::main
             at ./src/main.rs:4:5
  16: core::ops::function::FnOnce::call_once
             at /rustc/45e2c2881d11324d610815bfff097e25c412199e/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Mahmoud Al-Qudsi]

Hi there! I've verified the same issue and added a failing test to the repo. I've also opened PR #3 with a proposed patch for this. Could you try the code from #3 and see if it resolves the issue for you (and produces a valid result)?