fix(model): prevent division by zero in numHashers (#9744)

This should prevent the panic that occurred in this test run:
https://github.com/syncthing/syncthing/actions/runs/11095876010/job/30825046810

```
2024-09-29T21:01:53.5425372Z === RUN   TestIssue4357
2024-09-29T21:01:53.5505943Z panic: runtime error: integer divide by zero [recovered]
2024-09-29T21:01:53.5512200Z 	panic: runtime error: integer divide by zero
2024-09-29T21:01:53.5516633Z
2024-09-29T21:01:53.5523018Z goroutine 2655 [running]:
2024-09-29T21:01:53.5524157Z github.com/thejerf/suture/v4.(*Supervisor).runService.func2.2()
2024-09-29T21:01:53.5527176Z 	/home/runner/go/pkg/mod/github.com/thejerf/suture/[email protected]/supervisor.go:563 +0xd0
2024-09-29T21:01:53.5530556Z panic({0x1080d20?, 0x1851290?})
2024-09-29T21:01:53.5564723Z 	/home/runner/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:785 +0x132
2024-09-29T21:01:53.5566616Z github.com/syncthing/syncthing/lib/model.(*model).numHashers(0xc0006f6180, {0x117dc1a, 0x7})
2024-09-29T21:01:53.5568061Z 	/home/runner/work/syncthing/syncthing/lib/model/model.go:2581 +0x210
2024-09-29T21:01:53.5569912Z github.com/syncthing/syncthing/lib/model.(*folder).scanSubdirsChangedAndNew(0xc00c38c808, {0x0, 0x0, 0x0}, 0xc0003fc060)
2024-09-29T21:01:53.5571612Z 	/home/runner/work/syncthing/syncthing/lib/model/folder.go:653 +0x250
2024-09-29T21:01:53.5573010Z github.com/syncthing/syncthing/lib/model.(*folder).scanSubdirs(0xc00c38c808, {0x0, 0x0, 0x0})
2024-09-29T21:01:53.5574447Z 	/home/runner/work/syncthing/syncthing/lib/model/folder.go:512 +0xd0f
2024-09-29T21:01:53.5576011Z github.com/syncthing/syncthing/lib/model.(*folder).scanTimerFired(0xc00c38c808)
2024-09-29T21:01:53.5577367Z 	/home/runner/work/syncthing/syncthing/lib/model/folder.go:916 +0x46
2024-09-29T21:01:53.5579010Z github.com/syncthing/syncthing/lib/model.(*folder).Serve(0xc00c38c808, {0x1307650, 0xc0006a0910})
2024-09-29T21:01:53.5580428Z 	/home/runner/work/syncthing/syncthing/lib/model/folder.go:205 +0xd7e
2024-09-29T21:01:53.5581624Z github.com/thejerf/suture/v4.(*Supervisor).runService.func2()
2024-09-29T21:01:53.5582978Z 	/home/runner/go/pkg/mod/github.com/thejerf/suture/[email protected]/supervisor.go:567 +0x249
2024-09-29T21:01:53.5584400Z created by github.com/thejerf/suture/v4.(*Supervisor).runService in goroutine 2651
2024-09-29T21:01:53.5585872Z 	/home/runner/go/pkg/mod/github.com/thejerf/suture/[email protected]/supervisor.go:541 +0x32a
2024-09-29T21:01:53.5661413Z FAIL	github.com/syncthing/syncthing/lib/model	5.510s
```

### Testing

I have not been able to reproduce the panic throughout a few minutes of
continuously running the test without this fix, but judging by the
traceback it seems to only happen if the test happens to delete the
folder from config at the same time `scanTimerFired` triggers.

lib/model/model.go

@@ -2561,7 +2561,7 @@ func (m *model) DelayScan(folder string, next time.Duration) {
 func (m *model) numHashers(folder string) int {
 	m.mut.RLock()
 	folderCfg := m.folderCfgs[folder]
-	numFolders := len(m.folderCfgs)
+	numFolders := max(1, len(m.folderCfgs))
 	m.mut.RUnlock()
 
 	if folderCfg.Hashers > 0 {