tools/cgset: fix Uninitialized pointers read

tools/cgset: fix Uninitialized pointers read

Fix Uninitialized pointer read, reported by Coverity Tool:

CID 307778 (#2 of 2): Uninitialized pointer read (UNINIT)22.
uninit_use_in_call: Using uninitialized value src_cgroup when calling
cgroup_free.

In main(), src_cgrp points to a valid struct cgroup only when the user
calls cgset using --copy-from flag, Coverity warns about the freeing
src_cgrp, which is uninitialized in the error path.  This patch
initializes src_cgrp and cgroup uninitialized pointers and checks
is src_cgrp is valid pointer before passing it to cgroup_free()

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>