Fix: sink.text.pretty: Unsigned compared against 0

Fix: sink.text.pretty: Unsigned compared against 0

  CID 1401251 (#1 of 1): Unsigned compared against 0 (NO_EFFECT)
  unsigned_compare: This less-than-zero comparison of an unsigned value is
  never true. nr_fields < 0UL.

Reported-by: Coverity (1401251) Unsigned compared against 0
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8aa8a03a6b03ca1640570d70a51729d001af3b66
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1263
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>