intel: Return early if miptree allocation fails

If allocation fails in intel_miptree_create_internl(), don't proceed to
dereference the miptree. Return an early NULL.

Fixes static analysis error reported by Klocwork.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
(backport of commit 6b676e66343356d5096260c2bc1601014c808cb0)