Suppress Clang Static Analyzer warning in vdev_split()

Clang's static analyzer pointed out that we can have a NULL pointer
dereference if we ever attempt to split a vdev that has only 1 child. If
that happens, we are left with zero children, but then try to access a
non-existent child. Calling vdev_split() on a vdev with only 1 child
should be impossible due to how the code is structured. If this ever
happens, it would be best to stop execution immediately even in a
production environment to allow for the best possible chance of recovery
by an expert, so we use `VERIFY3U()` instead of `ASSERT3U()`.

Unfortunately, while that defensive assertion will prevent execution
from ever reaching the NULL pointer dereference, Clang's static analyzer
does not realize that, so we add an `ASSERT()` to inform it of this.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Richard Yao <[email protected]>
Closes openzfs#14575

module/zfs/vdev.c

@@ -5414,9 +5414,13 @@ vdev_split(vdev_t *vd)
 {
 	vdev_t *cvd, *pvd = vd->vdev_parent;
 
+	VERIFY3U(pvd->vdev_children, >, 1);
+
 	vdev_remove_child(pvd, vd);
 	vdev_compact_children(pvd);
 
+	ASSERT3P(pvd->vdev_child, !=, NULL);
+
 	cvd = pvd->vdev_child[0];
 	if (pvd->vdev_children == 1) {
 		vdev_remove_parent(cvd);