tools/cgget: fix segfault in get_cv_value()

tools/cgget: fix segfault in get_cv_value()

cgget segfaulted on v2.0.2 branch, with:
cgget: cannot find controller 'incal' in group '016cgget'
Fatal error: glibc detected an invalid stdio handle
Aborted (core dumped)

It was caught by ftests/016-cgget-invalid_options.py on Ubuntu 22.04, a
simple reproducer on the v2.0.2 branch:
$ sudo ./src/tools/cgget -n -v -r invalid.setting 016cgget
assuming 016cgget cgroup exists.

It is due to the invalid controller name passed to the
cgroup_read_value_begin(), which returns failure and callee
get_cv_value() in the error clean up path, does a fclose(handle).
If (handle != NULL) succeeds because its uninitialized and has some
garbage value. Fix this by initializing the handle to NULL.

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