[Rd] Long vectors: Missing values and R_xlen_t?

[Henrik Bengtsson]

Is there a missing value constant defined for R_xlen_t, cf. NA_INTEGER
(== R_NaInt == INT_MIN) for int(eger)?  If not, is it correct to
assume that missing values should be taken care/tested for before
coercing from int or double?

Thank you,

Henrik