vz: Before calling stopUsernet, ensure it’s not nil.

[norio-nomura]

Fix crashing on using external usernet:

$ limactl restart docker --progress
INFO[0000] Sending SIGINT to hostagent process 83134    
INFO[0000] Waiting for the host agent and the driver processes to shut down 
INFO[0000] [hostagent] Received SIGINT, shutting down the host agent 
INFO[0000] [hostagent] Shutting down the host agent     
INFO[0000] [hostagent] Shutting down VZ                 
INFO[0000] [hostagent] [VZ] - vm state change: stopped  
INFO[0001] [hostagent] panic: runtime error: invalid memory address or nil pointer dereference 
INFO[0001] [hostagent] [signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10117ee7c] 
INFO[0001] [hostagent] goroutine 49 [running]:          
INFO[0001] [hostagent] github.com/lima-vm/lima/v2/pkg/driver/vz.startVM.func2() 
INFO[0001] [hostagent] 	/Users/norio/ghq/github.com/lima-vm/lima/pkg/driver/vz/vm_darwin.go:146 +0x47c 
INFO[0001] [hostagent] created by github.com/lima-vm/lima/v2/pkg/driver/vz.startVM in goroutine 1 
INFO[0001] [hostagent] 	/Users/norio/ghq/github.com/lima-vm/lima/pkg/driver/vz/vm_darwin.go:81 +0x190 

introduced on #3949

[jandubois]

Thanks, LGTM

[norio-nomura]

Thanks! 🙏🏻