Fix potential NULL pointer dereference regression

Fix potential NULL pointer dereference regression

945b407486a0072ec7dd117a0bde2f72d52eb445 neglected to `NULL` check
`tx->tx_objset`, which is already done in the function. This upset
Coverity, which complained about a "dereference after null check".

Upon inspection, it was found that whenever `dmu_tx_create_dd()` is
called followed by `dmu_tx_assign()`, such as in
`dsl_sync_task_common()`, `tx->tx_objset` will be `NULL`.

Reported-by: Coverity (CID 1527261)
Reviewed-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Reviewed-by: Youzhong Yang <yyang@mathworks.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14170