rtld-elf: Fix leaks and wild frees in origin_subst

55abf23dd36b inverted the value passed to origin_subst_one when rolling
up the existing code into a loop. If the first token is found ($ORIGIN),
this results in a wild free of part of strtab. Processing the second
token works fine and will act how the first should have regardless of
whether found, allocating memory for the string without freeing.
Processing subsequent tokens however will then leak, regardless of
whether found, as they will also believe they need to allocate memory
and can't free the string.

Found by: CHERI
Reviewed by: kib, markj
Fixes: 55abf23dd36b ("rtld: make token substitution table-driven")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D35792

(cherry picked from commit becd9908beb8f1b47ddc6628cb005185a26ec85c)