Fix segfault/heap buffer overflow in `{Experimental,}DatasetToTFRecord` where dataset is numeric.

Code assumes only strings inputs and then interprets numbers as valid `tstring`s. Then, when trying to compute the CRC of the record this results in heap buffer overflow.

PiperOrigin-RevId: 387675909
Change-Id: I7396b9b8afc1ac744112af7c0b1cd7bb41e0f556