rtsold: fix memory leak in script execution

Since commit 04e9edb54428, rtsold has leaked the memory for the
argument vector of every script it runs.

Reported by: Coverity
Reviewed by: markj
Fixes: 04e9edb5442826a14616157962361ff81e4a38fe
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34355