Fix panic in IsSixelSupported

[cbednarski]

Hello!

I have been using qrterminal for some time and it is great! Recently I updated to 3.2.1 and I am experiencing a panic on Windows Terminal with Git Bash.

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x5c22df]

goroutine 1 [running]:
golang.org/x/term.restore(...)
        C:/Users/cbednarski/go/pkg/mod/golang.org/x/term@v0.32.0/term_windows.go:45
golang.org/x/term.Restore(...)
        C:/Users/cbednarski/go/pkg/mod/golang.org/x/term@v0.32.0/term.go:45
github.com/mdp/qrterminal/v3.IsSixelSupported({0x9f9740?, 0xc000088060?})
        C:/Users/cbednarski/go/pkg/mod/github.com/mdp/qrterminal/v3@v3.2.1/qrterminal.go:70 +0x1b0
github.com/mdp/qrterminal/v3.Generate({0xc000484100, 0x19}, 0xc0002add20?, {0x9f9740?, 0xc000088060?})
        C:/Users/cbednarski/go/pkg/mod/github.com/mdp/qrterminal/v3@v3.2.1/qrterminal.go:247 +0x93
main.PreviewCommand(0xc0002c53c0)

This is caused by an unhandled error in IsSixelSupported.

	raw, err := term.MakeRaw(int(stdout.Fd()))  <-- unhandled error
	defer term.Restore(int(stdout.Fd()), raw)   <-- panics when the function returns

This PR adds the missing error handling.

I am not sure whether it is feasible to add a test since it is environment specific, but please let me know if you would like any further changes. Thanks!

[cbednarski]

Hi there @mdp, I'm sure you're quite busy with other things but I would appreciate a moment of your time to take a look here, if you're able to spare one! Cheers!