update PCR_error detection

lib/tssmooth.js

@@ -76,7 +76,7 @@ function TsSmooth(options) {
     }
 
     var delta = pcrtime - self.pcrtime;
-    if (delta > 100E3 || delta < -500E3) {
+    if (delta > 100E3 || delta < 0) {
       console.error('PCR_error: '+(delta/1E6).toFixed(2)+'s missing');
       var now = utime();
       var error = now - pcrtime;