Add support for encoding/decoding fixed data

This patch add fixed data support to the C implementation

Just as with string and bytes processing, I have added two new methods: avro_wrapfixed, avro_givefixed to allow developers that freedom to decide how the bytes of memory are managed. avro_wrapfixed will not free the bytes when the fixed datum is dereferenced. avro_givefixed will free the bytes memory when dereferenced. avro_fixed will make a copy of the bytes.

I also plugged a small potential memory leak issue that could occur because of IO errors.