pnmdec: drop frame in case of _handle() failure

Allows baseclass to handle it from there

Related to:

https://bugzilla.gnome.org/show_bug.cgi?id=756563

gst/pnm/gstpnmdec.c

@@ -228,6 +228,7 @@ gst_pnmdec_handle_frame (GstVideoDecoder * decoder, GstVideoCodecFrame * frame)
 
   r = gst_video_decoder_allocate_output_frame (decoder, frame);
   if (r != GST_FLOW_OK) {
+    gst_video_decoder_drop_frame (GST_VIDEO_DECODER (s), frame);
     goto out;
   }