src/libpcp/src/exec.c: free alloc'd storage in child on error
src/libpcp/src/exec.c: free alloc'd storage in child on error

Fixes qa/681 failures seen on some platforms when valgrind
detects a leak in the child process after it fails to exec().

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp