Suppress Clang Static Analyzer warning in dsl_dir_rename_sync()

Clang's static analyzer reports that if we try to rename a root dataset
in dsl_dir_rename_sync(), we will have a NULL pointer passed to
strlcpy(). This is impossible because dsl_dir_rename_check() will
prevent us from doing this. We add an assertion to silence this warning.

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