Fix: ctf: assert that name is not NULL in warn_meaningless_field()

Fix: ctf: assert that name is not NULL in warn_meaningless_field()

gcc 9.1 reports that warn_meaningless_field's `name` is sometimes
NULL when it is inlined in warn_meaningless_fields(), itself
inlined in ctf_trace_class_warn_meaningless_header_fields() at
line 131.

The the only call site using a `NULL` name will have
`fc->type == CTF_FIELD_CLASS_TYPE_STRUCT`.

Add a `BT_ASSERT(name)` to warn_meaningless_field() and a comment
indicating that `name` is guaranteed to be non-NULL whenever the field
class is not a structure.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I323b9f390f80dbd898a69664082fd07d5cc2fc86
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2000
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>