`SIZE_MAX` is not representable as `double`

[aitap]

Found by the lin-dev-clang-cran check:

freadR.c:835:21: warning: implicit conversion from 'unsigned long' to 'double' changes value from 18446744073709551615 to 18446744073709551616 [-Wimplicit-const-int-float-conversion]

This doesn't break anything right now, but it's a correctness issue.