Suppress static analyzer warning in dmu_objset_create_impl_dnstats()

ae7e7006500ca37c471dd625cd5cbfc590b49885 added an assertion to suppress
a complaint from Clang's static analyzer. Unfortunately, it missed
another way for Clang to complain about this function. This adds another
assertion to handle that.

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