v4l2codecs: h264: Reset the number of slices on picture start

This way, if we aborted a frame in the middle, we don't endup thinking
there is multiple slices to send.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1079>

subprojects/gst-plugins-bad/sys/v4l2codecs/gstv4l2codech264dec.c

@@ -984,6 +984,7 @@ gst_v4l2_codec_h264_dec_start_picture (GstH264Decoder * decoder,
       dpb);
 
   self->first_slice = TRUE;
+  self->num_slices = 0;
 
   return GST_FLOW_OK;
 }