Compile can panic when variables get ignored

In case a variable inside the Inventory points ot another variable which no longer exists, the ReplaceVariables function would remove those vars from the list of vars, see https://github.com/lukasjarosch/skipper/blob/80ea958e3bddf36fc76d5a1b9828e31679762534/variable.go#L207.

But because function overwrites the array but still iterates over the range of elements of the original array, the function could encounter an "index out of bounds" error.

panic: runtime error: index out of range [33] with length 32

goroutine 1 [running]:
github.com/lukasjarosch/skipper.ReplaceVariables(0x4000359140, {0x4000615300?, 0x6d2ad0?, 0xb?}, 0x0?)
	/bob/vendor/github.com/lukasjarosch/skipper/variable.go:206 +0x2d4
github.com/lukasjarosch/skipper.(*Inventory).Data(0x40000e0360, {0xffffe31e5e1d, 0x9}, 0x0, 0x0, 0x1)
	/bob/vendor/github.com/lukasjarosch/skipper/inventory.go:217 +0x438
main.main.func2(0x4000039100?)
	/bob/cmd/bob/main.go:132 +0x94
github.com/urfave/cli/v2.(*Command).Run(0x40001a6dc0, 0x4000039100, {0x4000039140, 0x4, 0x4})
	/bob/vendor/github.com/urfave/cli/v2/command.go:276 +0x600
github.com/urfave/cli/v2.(*Command).Run(0x40001a7600, 0x4000038f00, {0x400001e050, 0x5, 0x5})
	/bob/vendor/github.com/urfave/cli/v2/command.go:269 +0x83c
github.com/urfave/cli/v2.(*App).RunContext(0x400022a400, {0x7d7b50, 0xc2e540}, {0x400001e050, 0x5, 0x5})
	/bob/vendor/github.com/urfave/cli/v2/app.go:333 +0x51c
github.com/urfave/cli/v2.(*App).Run(...)
	/bob/vendor/github.com/urfave/cli/v2/app.go:307
main.main()
	/bob/cmd/bob/main.go:281 +0x1090

To Reproduce
In your inventory, simply add a reference to a var that does not exist.

Start in variable.go at ReplaceVariables, then reproduce through Inventory.Data using an inventory reference to a missing variable. Done means that input no longer panics with an index-out-of-bounds error while variable replacement completes.