Add a lua interface for the C API snappy_validate_compressed_buffer

The C API snappy_validate_compressed_buffer (snappy/snappy-c.h, line 131)
```snappy_status snappy_validate_compressed_buffer(const char* compressed, size_t compressed_length);```
is used for checking if the contents of "compressed[]" can be uncompressed successfully. 
This API might been used before uncompress the buffered bytes .