Fix potential log panic (#82)

If a job uses a [reusable workflow](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-of-jobsjob_iduses), the job's steps sequence will be empty.

But in log reporter, we don't check the length of `r.state.Steps`, which may cause panic.

``` go
if v, ok := entry.Data["stepNumber"]; ok {
	if v, ok := v.(int); ok {
		step = r.state.Steps[v]
	}
}
```

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/82
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

runtime/reporter.go

@@ -99,7 +99,7 @@ func (r *Reporter) Fire(entry *log.Entry) error {
 
 	var step *runnerv1.StepState
 	if v, ok := entry.Data["stepNumber"]; ok {
-		if v, ok := v.(int); ok {
+		if v, ok := v.(int); ok && len(r.state.Steps) > v {
 			step = r.state.Steps[v]
 		}
 	}