rtld: fix after r356300

binpath local was changed from char array to a char pointer, update
strlcpy/strlcat uses.

Reported by: Coverity through vangyzen
CID: 1412239 and 1412240
Reviewed by: emaste, imp, vangyzen
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23090