Panic on a mode having no patterns

[nihei9]

When a mode contains no patterns, not fragments, maleeni v0.6.0 panics.

I made the following file:

{
	"name": "test",
	"entries": [
		{
			"modes": ["other_mode"],
			"kind": "foo",
			"pattern": "foo"
		}
	]
}

Then I got the following error:

$ maleeni compile test.json
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x5b2510]

goroutine 1 [running]:
github.com/nihei9/maleeni/compiler/dfa.GenDFA(0x0, 0x0, 0xc000118dd0, 0xc000118dd0)
        /home/nihei9/src/github.com/nihei9/maleeni/compiler/dfa/dfa.go:51 +0x50
github.com/nihei9/maleeni/compiler.compile(0x832280, 0x0, 0x0, 0xc00011b350, 0xc00011b3b0, 0xc0001a4f78, 0xc00014c400, 0x3, 0x4, 0xc00011b350, ...)
        /home/nihei9/src/github.com/nihei9/maleeni/compiler/compiler.go:323 +0x1b7d
github.com/nihei9/maleeni/compiler.Compile(0xc00011b050, 0xc0001add18, 0x1, 0x1, 0x0, 0xc0001b8100, 0xc0001add10, 0xc0001add38, 0xc0001add38, 0x5cd065)
        /home/nihei9/src/github.com/nihei9/maleeni/compiler/compiler.go:57 +0x9a7
main.runCompile(0xc000154f00, 0xc000118ad0, 0x1, 0x1, 0x0, 0x0)
        /home/nihei9/src/github.com/nihei9/maleeni/cmd/maleeni/compile.go:49 +0xe5
github.com/spf13/cobra.(*Command).execute(0xc000154f00, 0xc000118ab0, 0x1, 0x1, 0xc000154f00, 0xc000118ab0)
        /home/nihei9/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:852 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0x800100, 0x4407dc, 0xc000000180, 0x300000002)
        /home/nihei9/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:960 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
        /home/nihei9/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:897
main.Execute(0x7e9d88, 0xc00010e058)
        /home/nihei9/src/github.com/nihei9/maleeni/cmd/maleeni/root.go:22 +0x31
main.main()
        /home/nihei9/src/github.com/nihei9/maleeni/cmd/maleeni/main.go:8 +0x25

The following spec causes the same error.

{
	"name": "test",
	"entries": [
		{
			"kind": "foo",
			"fragment": true,
			"pattern": "foo"
		}
	]
}