Fix a race condition where Camera2Session forgot to return the texture frame when closing the camera.

BUG=b/31201445

Review-Url: https://codereview.webrtc.org/2289303005
Cr-Commit-Position: refs/heads/master@{#14015}

webrtc/api/android/java/src/org/webrtc/Camera2Session.java

@@ -176,6 +176,7 @@ public class Camera2Session implements CameraSession {
 
               if (state != SessionState.RUNNING) {
                 Logging.d(TAG, "Texture frame captured but camera is no longer running.");
+                surfaceTextureHelper.returnTextureFrame();
                 return;
               }