Fix "high impact" Coverity error 1212392
[trivial, not reviewed]

There were two places in arg.c that failed to free allocated strings.  Free
them before they go out of scope.

Tested on full standard tests.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23617 3a8e244f-b0f2-452b-bcba-4c88e055c3ca