Prevent double-free when y is in array t

[RSilicon]

The code already accounts for when y is in frp->args, but not when it is in frp->args->csub. This PR changes that by adding a function that frees while checking to see if y was found in the array and subsequently freed.

[plan9]

we will not be introducing new code at this point. we may revisit this issue.

[millert]

This is still an issue and results in a use-after-free under both valgrind and -fsanitize=address when awk is given the following input from T.misc:

BEGIN { unireghf() }
function unireghf(hfeed) { hfeed[1] = 0 }