Fix divide by zero in common.DemoHeader.FrameTime (#109)

* Return 0 in common.DemoHeader.FrameTime if PlaybackFrames is 0 to avoid dividing by zero

Co-Authored-By: marksamman <mark.samman@gmail.com>

Author: marksamman

common/common.go

@@ -41,7 +41,12 @@ func (h DemoHeader) FrameRate() float64 {
 }
 
 // FrameTime returns the time a frame / demo-tick takes in seconds.
+//
+// Returns 0 if PlaybackTime or PlaybackFrames are 0 (corrupt demo headers).
 func (h DemoHeader) FrameTime() time.Duration {
+	if h.PlaybackFrames == 0 {
+		return 0
+	}
 	return time.Duration(h.PlaybackTime.Nanoseconds() / int64(h.PlaybackFrames))
 }