Empty files causes a panic

[hachi8833]

If Goby runs an empty file (or only LFs contained), the following panic causes:

$ goby 50sandbox.gb # empty file
panic: runtime error: index out of range

goroutine 1 [running]:
github.com/goby-lang/goby/compiler/bytecode.(*Generator).compileStatements(0xc420061d80, 0x48767c8, 0x0, 0x0, 0xc420089200, 0xc4200f4860)
	/Users/hachi8833/deve/golang/gopath/sys/src/github.com/goby-lang/goby/compiler/bytecode/statement_generation.go:25 +0x27b
github.com/goby-lang/goby/compiler/bytecode.(*Generator).GenerateInstructions(0xc420061d80, 0x48767c8, 0x0, 0x0, 0xc4200891d0, 0xc42000e030, 0xc420061db0)
	/Users/hachi8833/deve/golang/gopath/sys/src/github.com/goby-lang/goby/compiler/bytecode/generator.go:54 +0x5b
github.com/goby-lang/goby/compiler.CompileToInstructions(0x0, 0x0, 0x0, 0x200, 0x0, 0x0, 0x7ffeefbfe467, 0x2)
	/Users/hachi8833/deve/golang/gopath/sys/src/github.com/goby-lang/goby/compiler/compiler.go:34 +0x1e0
main.main()
	/Users/hachi8833/deve/golang/gopath/sys/src/github.com/goby-lang/goby/goby.go:60 +0x303

Looks like the issue is related to #560.