Panic with incorrect input

[johejo]

How to reproduce

package errorformat_test

import (
	"testing"

	"github.com/reviewdog/errorformat"
)

func TestPanic(t *testing.T) {
	_, err := errorformat.NewEfm("%")
	if err == nil {
		t.Fatal("should return error")
	}
}

--- FAIL: TestPanic (0.00s)
panic: runtime error: index out of range [1] with length 1 [recovered]
        panic: runtime error: index out of range [1] with length 1

goroutine 33 [running]:
testing.tRunner.func1.1(0x5406c0, 0xc000154020)
        /home/linuxbrew/.linuxbrew/Cellar/go/1.15.7_1/libexec/src/testing/testing.go:1072 +0x30d
testing.tRunner.func1(0xc000102480)
        /home/linuxbrew/.linuxbrew/Cellar/go/1.15.7_1/libexec/src/testing/testing.go:1075 +0x41a
panic(0x5406c0, 0xc000154020)
        /home/linuxbrew/.linuxbrew/Cellar/go/1.15.7_1/libexec/src/runtime/panic.go:969 +0x1b9
github.com/reviewdog/errorformat.NewEfm.func1(...)
        /home/heijo/ghq/github.com/reviewdog/errorformat/errorformat.go:435
github.com/reviewdog/errorformat.NewEfm(0x54feb1, 0x1, 0xc00005e770, 0x47e246, 0x60281766)
        /home/heijo/ghq/github.com/reviewdog/errorformat/errorformat.go:442 +0x93f
github.com/reviewdog/errorformat_test.TestPanic(0xc000102480)
        /home/heijo/ghq/github.com/reviewdog/errorformat/panic_test.go:10 +0x3a
testing.tRunner(0xc000102480, 0x55b548)
        /home/linuxbrew/.linuxbrew/Cellar/go/1.15.7_1/libexec/src/testing/testing.go:1123 +0xef
created by testing.(*T).Run
        /home/linuxbrew/.linuxbrew/Cellar/go/1.15.7_1/libexec/src/testing/testing.go:1168 +0x2b3
exit status 2
FAIL    github.com/reviewdog/errorformat        0.004s