rule ~x (tilde x) makes R-session crash for logical column x

[flownt]

Consider

library(validate)
tt <- data.frame(x=c(T, T))
vld <- validator( ~x)

confront(tt, vld)

I am not sure what the result of ~x should be, however a crash of the complete R-session is not an acceptable result.

Tested on validate 1.1.5