tab can't return frequencies for P/A genpop objects

[zkamvar]

Example:

x <- read.table(system.file("files/AFLP.txt", package = "adegenet"))
aflp <- df2genind(x, type = "PA", ploidy = 1, pop = c(rep(1, 4), rep(2, 3)))
tab(genind2genpop(aflp), freq = TRUE)
## Error in FUN(newX[, i], ...) : arguments must have same length 

This comes down to the line here where it attempts to do a tapply using a NULL loc.fac slot. I'll take care of this bug and write a test.

[thibautjombart]

thanks for the fix!