[bug] `tailcat genkey --embed-derp-map` cause panic

[chanchiwai-ray]

When running tailcat genkey --key=test-key --embed-derp-map without --region or with --region=auto (the default value) cause panic.

$ tailcat genkey --key=test-key --embed-derp-map
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x48 pc=0xb228c8]

goroutine 1 [running]:
main.genKey({0x248f247840b0?, 0x248f247840a0?, 0xb9caa9?})
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:1692 +0xfc8
main.newRootCommand.func7({0x0?, 0x0?}, {0x248f247840b0?, 0x6?, 0x248f24790a00?})
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:222 +0x25
github.com/peterbourgon/ff/v4.(*Command).Run(0x248f2479ab50?, {0x14d3bf8?, 0x1575ea0?})
	/root/go/pkg/mod/github.com/peterbourgon/ff/v4@v4.0.0-beta.1/command.go:146 +0x132
github.com/peterbourgon/ff/v4.(*Command).Run(0x248f24790c80?, {0x14d3bf8?, 0x1575ea0?})
	/root/go/pkg/mod/github.com/peterbourgon/ff/v4@v4.0.0-beta.1/command.go:148 +0x145
main.main()
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:600 +0xf8

or

$ tailcat genkey --key=test-key --embed-derp-map --region=auto 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x48 pc=0xb228c8]

goroutine 1 [running]:
main.genKey({0x36b13591c230?, 0x36b13591c200?, 0xb9caa9?})
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:1692 +0xfc8
main.newRootCommand.func7({0x0?, 0x0?}, {0x36b13591c230?, 0x6?, 0x36b1359ec3c0?})
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:222 +0x25
github.com/peterbourgon/ff/v4.(*Command).Run(0x36b1359aa7f0?, {0x14d3bf8?, 0x1575ea0?})
	/root/go/pkg/mod/github.com/peterbourgon/ff/v4@v4.0.0-beta.1/command.go:146 +0x132
github.com/peterbourgon/ff/v4.(*Command).Run(0x36b1359ec640?, {0x14d3bf8?, 0x1575ea0?})
	/root/go/pkg/mod/github.com/peterbourgon/ff/v4@v4.0.0-beta.1/command.go:148 +0x145
main.main()
	/root/parts/tailcat/build/cmd/tailcat/tailcat.go:600 +0xf8

tailcat version: v0.5.0