Silence clang static analyzer warnings about stored stack addresses

Clang's static analyzer complains that nvs_xdr() and nvs_native()
functions return pointers to stack memory. That is technically true, but
the pointers are stored in stack memory from the caller's stack frame,
are not read by the caller and are deallocated when the caller returns,
so this is harmless. We set the pointers to NULL to silence the
warnings.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14612