Add support for zlib -1 compression level

[McTwist]

zlib supports -1 as a compression level, basically setting the compression to 6 (current default). It should be straightforward in libdeflate_alloc_compressor_ex:

if (compression_level == -1)
    compression_level = 6;

This way the library will define the default, and the user can rely on that default.

[n3rn0]

diff --git a/lib/deflate_compress.c b/lib/deflate_compress.c
index 4a1f327..916f125 100644
--- a/lib/deflate_compress.c
+++ b/lib/deflate_compress.c
@@ -3885,6 +3885,9 @@ libdeflate_alloc_compressor_ex(int compression_level,
        if (options->sizeof_options != sizeof(*options))
                return NULL;
 
+       if (compression_level == -1)
+               compression_level = 6;
+
        if (compression_level < 0 || compression_level > 12)
                return NULL;

[tansy]

You may want to make a pull request.