zdb: Fix minor memory leak

Commit 6b6aaf6dc2e65c63c74fbd7840c14627e9a91ce2 introduced a small
memory leak in zdb. This was detected by the LeakSanitizer and was
causing all ztest runs to fail.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14796