[bug] fread fails to read single-column file with fill=TRUE

[st-pasha]

Minimally reproducible example:

fread("1\n2\n3", fill=T)
# Error in fread("1\n2\n3", fill = T) : 
#   Internal error: ncol==0 line==1 after detecting sep, ncol and first line

There is a related bug (trivial to fix) there, in the error-reporting line:

STOP("Internal error: ncol==%d line==%d after detecting sep, ncol and first line");

here ncol and line are not passed as the varargs, which may result in random data being printed, or occasionally segfaulting.