Remove unnecessary local variable copy.

It is only set to null in Run, protected by mutex_.
It is only set to non null in Unregister, protected by reactable->mutex_

So if we're holding mutex_, it can only transition to non-null.

Meaning if we check it an it is not null, it cannot change.

Test: gd/fuzzing/run --host bluetooth_gd_hci_fuzz_test
Change-Id: Iea2dbc1e050531408ca4a153f088b0dbf8f6cbd7