Unreachable code in thumbhash_encoder_create function

[skidder]

The thumbhash_encoder_create function includes a null-check on the return value from new; however, if new were to fail, the default behavior is to throw an std::bad_alloc exception rather than return null, making the if-condition unreachable.

https://github.com/discord/lilliput/blob/217a9d5cff47a3310851f1f9239df962c5b2c1bd/thumbhash.cpp#L18-L21