fix crash

internal/core/path.go

@@ -524,9 +524,11 @@ func (pa *path) externalCmdEnv() externalcmd.Environment {
 		"RTSP_PORT": port,
 	}
 
+	if len(pa.matches) > 1 {
 		for i, ma := range pa.matches[1:] {
 			env[strconv.FormatInt(int64(i+1), 10)] = ma
 		}
+	}
 
 	return env
 }