Fix: evaluation: dereference before NULL check in create_from_payload

An evaluation payload view is created from the view passed to
lttng_evaluation_create_from_payload. Since a view contains a const
copy of the _fds array, it must be initialized as the declaration site.

However, src_view is checked for NULL after the initalization. Coverity
rightfully warns that:

  1429799 Dereference before null check

  There may be a null pointer dereference, or else the comparison against
  null is unnecessary.

  In lttng_evaluation_create_from_payload: All paths that lead to this
  null pointer comparison already dereference the pointer
  earlier (CWE-476)

This is not reachable right now, but it is fixed to silence the warning
and prevent future mistakes.

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3ceace4117ff54265b1f0cf6a4c638aec95f2879