Optimize EOF checking in hed_file_set_byte

Optimize EOF checking in hed_file_set_byte

Instead of comparing the position, check the EOF flag of the following
block. Note that I can't use the current block, because that is already
devirtualized when I check, and I cannot check earlier, because
prepare_modify() may fail and I would have to roll back.

Note that next_block() must always return a valid block pointer here.