datum: Wrap ord() around initial next_byte() call
datum: Wrap ord() around initial next_byte() call

Assigning it to a variable increases the risk of it being used
somewhere without ord().

This is purely a style change following dfe0ddc, to make sure
skip_long() is consistent with all other uses of next_byte() in
this class, which already wrap ord() around the call.

Change-Id: I57e9864f4f458dccf482e1e20cad8c6b7eed5f45

wikimedia/avro-php