Ensure that 'castCall' is not "may be used uninitialized" in wrappers.cp...

I did not reproduce the original error, so not sure this is a fix. From visual observation, it should be.

The original error occurred according to this job:

Hostname: chap12
Revision: https://github.com/chapel-lang/chapel/commit/4d052f3 [github.com]
Logfile:  /tmp/day5-Fri-linux64.gnu.none.qthreads.flat-fst.log.18419
Started:  Fri Aug 29 02:08:28 2014
Ended:    Fri Aug 29 02:09:47 2014
        http://chapel-ci.us.cray.com/job/correctness-chap12/77/