FreeBSD: Fix uninitialized pointer read in spa_import_rootpool()
The FreeBSD project's coverity scans found this.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13923