Fix: sink.ctf.fs: Dereference before null check

Fix: sink.ctf.fs: Dereference before null check

No need to check if `trace->metadata_path` is null at the point because
it has been used by `fopen()` already and `abort()` is called if
`fopen()` fails.

  CID 1401238 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking trace->metadata_path suggests that it
  may be null, but it has already been dereferenced on all paths leading
  to the check.

Reported-by: Coverity (1401238) Dereference before null check
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I90590323764810da8a6b4cfb37445a588befd242
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1284
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tested-by: jenkins