[MOVED FROM BAD 121/134] vp8enc: initiate encoder to fix a crash.

Without this patch vp8enc send header before and after first
key frame. On second keyframe vp8dec will crash without getting
decoded frame. With this pipe it is easy to reproduce this issue:
gst-launch-1.0 videotestsrc ! vp8enc ! vp8dec ! fakesink

https://bugzilla.gnome.org/show_bug.cgi?id=680667

ext/vp8/gstvp8enc.c

@@ -941,6 +941,8 @@ gst_vp8_enc_set_format (GstVideoEncoder * video_encoder,
       gst_video_encoder_set_output_state (video_encoder, caps, state);
   gst_video_codec_state_unref (output_state);
 
+  gst_video_encoder_negotiate (GST_VIDEO_ENCODER (encoder));
+
   return ret;
 }