Check for nil when loading atomic values

This is required to prevent an error when values are loaded
without first being set.

README.md

@@ -47,6 +47,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [Chao Yuan](https://github.com/yuanchao0310)
 * [Jason Maldonis](https://github.com/jjmaldonis)
 * [Nevio Vesic](https://github.com/0x19)
+* [David Hamilton](https://github.com/dihamilton)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

candidate_base.go

@@ -194,7 +194,11 @@ func (c *candidateBase) String() string {
 // LastReceived returns a time.Time indicating the last time
 // this candidate was received
 func (c *candidateBase) LastReceived() time.Time {
-	return c.lastReceived.Load().(time.Time)
+	lastReceived := c.lastReceived.Load()
+	if lastReceived == nil {
+		return time.Time{}
+	}
+	return lastReceived.(time.Time)
 }
 
 func (c *candidateBase) setLastReceived(t time.Time) {
@@ -204,7 +208,11 @@ func (c *candidateBase) setLastReceived(t time.Time) {
 // LastSent returns a time.Time indicating the last time
 // this candidate was sent
 func (c *candidateBase) LastSent() time.Time {
-	return c.lastSent.Load().(time.Time)
+	lastSent := c.lastSent.Load()
+	if lastSent == nil {
+		return time.Time{}
+	}
+	return lastSent.(time.Time)
 }
 
 func (c *candidateBase) setLastSent(t time.Time) {