Fix: sink.utils.counter: possible NULL pointer dereference

If the null check at line 148 is true, the `counter` pointer might be
used will be used while being NULL by the
`destroy_private_counter_data()` function.

scan-build report:
  Access to field 'msg_iter' results in a dereference of a null pointer
  (loaded from variable 'counter')

Reported-by: scan-build - Logic error Dereference of null pointer
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I50bb981433ef3f5083613b36bbef54743cddb4c2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1970
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>