Cleanup: spa vdev processing should check NULL pointers

The PVS Studio 2016 FreeBSD kernel report stated:

\contrib\opensolaris\uts\common\fs\zfs\spa.c (1341): error V595: The 'spa->spa_spares.sav_vdevs' pointer was utilized before it was verified against nullptr. Check lines: 1341, 1342.
\sys\cddl\contrib\opensolaris\uts\common\fs\zfs\spa.c (1355): error V595: The 'spa->spa_l2cache.sav_vdevs' pointer was utilized before it was verified against nullptr. Check lines: 1355, 1357.
\sys\cddl\contrib\opensolaris\uts\common\fs\zfs\spa.c (1398): error V595: The 'spa->spa_spares.sav_vdevs' pointer was utilized before it was verified against nullptr. Check lines: 1398, 1408.
\sys\cddl\contrib\opensolaris\uts\common\fs\zfs\spa.c (1583): error V595: The 'oldvdevs' pointer was utilized before it was verified against nullptr. Check lines: 1583, 1595.

In practice, all of these uses were safe because a NULL pointer
implied a 0 vdev count, which kept us from iterating over vdevs.
However, rearranging the code to check the pointer first is not a
terrible micro-optimization and makes it more readable, so let us
do that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14456