Index recycled string in str_dup() sep branch

[sims1253]

str_dup() in R/dup.R recycles string and times to a common length with vctrs::vec_recycle_common() and stores the result in input. The sep code path loops over seq_along(input$string) but reads elements from the original string, so it walks past the end whenever times is longer than string:

str_dup("na", 0:5, sep = "-")
#> Error: subscript out of bounds

The same call without sep works, because that path hands input$string and input$times to stri_dup(). This PR fixes the index in the sep loop.

str_dup("na", 0:5, sep = "-") now returns "", "na", "na-na", "na-na-na", "na-na-na-na", "na-na-na-na-na".

Found during my most recent ry audit.