Quick fix for a out of range error that could occur (#236)

This error could occur when a template has a skeleton referencing joints that are not present in template

com.unity.perception/CHANGELOG.md

 
 Fixed keypoints being reported in wrong locations on the first frame an object is visible. 
 
+Fixed an out of range error if a keypoint template skeleton relies on a joint that is not available.
+
 ## [0.7.0-preview.2] - 2021-02-08
 
 ### Upgrade Notes

com.unity.perception/Runtime/GroundTruth/Labelers/KeypointLabeler.cs

             return "unset";
         }
 
+        Keypoint GetKeypointForJoint(KeypointEntry entry, int joint)
+        {
+            if (joint < 0 || joint >= entry.keypoints.Length) return null;
+            return entry.keypoints[joint];
+        }
+
         /// <inheritdoc/>
         protected override void OnVisualize()
         {
             {
                 foreach (var bone in activeTemplate.skeleton)
                 {
-                    var joint1 = entry.keypoints[bone.joint1];
-                    var joint2 = entry.keypoints[bone.joint2];
+                    var joint1 = GetKeypointForJoint(entry, bone.joint1);
+                    var joint2 = GetKeypointForJoint(entry, bone.joint2);
-                    if (joint1.state == 2 && joint2.state == 2)
+                    if (joint1 != null && joint1.state == 2 && joint2 != null && joint2.state == 2)
                     {
                         VisualizationHelper.DrawLine(joint1.x, joint1.y, joint2.x, joint2.y, bone.color, 8, skeletonTexture);
                     }