fix: Accessing fctx->state without holding lock

Move lock earlier in the call sequence to address access without lock report.

```
1559        /*
1560         * Caller must be holding the fctx lock.
1561         */

CID 468796: (#1 of 1): Data race condition (MISSING_LOCK)
1. missing_lock: Accessing fctx->state without holding lock fetchctx.lock. Elsewhere, fetchctx.state is written to with fetchctx.lock held 2 out of 2 times.
1562        REQUIRE(fctx->state == fetchstate_done);
1563
1564        FCTXTRACE("sendevents");
1565
1566        LOCK(&fctx->lock);
1567
```

Closes #4902

Merge branch '4902-accessing-fctx-state-without-holding-lock' into 'main'

Closes #4902

See merge request isc-projects/bind9!9427