ruleguard panics when Suggest'ing one + N arguments

[tdakkota]

To reproduce

func BadRule(m dsl.Matcher) {
	m.Match("fmt.$method($arg, $*args)").Suggest("log.$method($arg, $args)")
}

Original rule for zap

package gorules

import (
  "github.com/quasilyte/go-ruleguard/dsl"
)

func ZapPreferNoWith(m dsl.Matcher) {
  m.Import("go.uber.org/zap")

  m.Match("$l.With($arg, $*args).$method($*msg_args)").Where(
  	m["l"].Type.Is("*zap.Logger") &&
  		m["method"].Text.Matches("Debug|Info|Warn|Error|DPanic|Panic|Fatal"),
  ).Suggest("$l.$method($msg_args, $arg, $args)")
}

Partial stacktrace

panic: runtime error: index out of range [0] with length 0

goroutine 3525 [running]:
github.com/quasilyte/go-ruleguard/internal/mvdan.cc/gogrep.exprList.Pos(...)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/internal/mvdan.cc/gogrep/match.go:1098
github.com/quasilyte/go-ruleguard/ruleguard.(*rulesRunner).nodeText(0xc010f1ab00, 0x509720, 0xc010d87ea0, 0x5, 0x34, 0xc00b7a8990)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/ruleguard/runner.go:57 +0x65
github.com/quasilyte/go-ruleguard/ruleguard.(*rulesRunner).renderMessage(0xc010f1ab00, 0xc0006836b0, 0x2e, 0x5083a0, 0xc00b7afd00, 0xc010f2cea0, 0xc010f1ab01, 0x508c60, 0x387888)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/ruleguard/runner.go:232 +0x472
github.com/quasilyte/go-ruleguard/ruleguard.(*rulesRunner).handleMatch(0xc010f1ab00, 0xc00b399f80, 0xf, 0xc0000a0160, 0x12, 0x14, 0xc010d87e20, 0xc0006836b0, 0x2e, 0x0, ...)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/ruleguard/runner.go:174 +0xf4
github.com/quasilyte/go-ruleguard/ruleguard.(*rulesRunner).run.func2.1(0x5083a0, 0xc00b7afd00, 0xc010f2cea0)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/ruleguard/runner.go:107 +0xe8
github.com/quasilyte/go-ruleguard/internal/mvdan.cc/gogrep.(*Pattern).MatchNode(0xc010d87e20, 0x5083a0, 0xc00b7afd00, 0xc00b7a8d78)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/internal/mvdan.cc/gogrep/kludge.go:50 +0xbb
github.com/quasilyte/go-ruleguard/ruleguard.(*rulesRunner).run.func2(0x5083a0, 0xc00b7afd00, 0x508801)
        C:/Users/tdakkota/go/pkg/mod/github.com/quasilyte/go-ruleguard@v0.3.1-0.20210125091851-f953b71a3ba3/ruleguard/runner.go:106 +0x19f
go/ast.inspector.Visit(0xc010de2c40, 0x5083a0, 0xc00b7afd00, 0x5069c0, 0xc010de2c40)
        c:/go/src/go/ast/walk.go:373 +0x42

[quasilyte]

It's simple to fix a panic, but a proper solution would require some changes to the gogrep.
I'll see what I can do.