Suppress clang static analyzer warning in vdev_stat_update()

63652e154643cfe596fe077c13de0e7be34dd863 added unnecessary branches in
vdev_stat_update() to suppress an ASAN false positive the breaks
ztest. This had the downside of causing false positive reports in both
Coverity and Clang's static analyzer. vd is never NULL, so we add a
preprocessor check to only apply the workaround when compiling with ASAN
support.

Reported-by: Coverity (CID-1524583)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14575