codecs: h265decoder: Fix per-slice leak

As documented, slice header parsed via gst_h265_parser_parse_slice_hdr()
should be cleared, otherwise it would result in memory leak.

Part-of: <!2559>

gst-libs/gst/codecs/gsth265decoder.c

@@ -380,6 +380,12 @@ gst_h265_decoder_parse_slice (GstH265Decoder * self, GstH265NalUnit * nalu,
     return FALSE;
   }
 
+  /* NOTE: gst_h265_parser_parse_slice_hdr() allocates array
+   * GstH265SliceHdr::entry_point_offset_minus1 but we don't use it
+   * in this h265decoder baseclass at the moment
+   */
+  gst_h265_slice_hdr_free (&priv->current_slice.header);
+
   priv->current_slice.nalu = *nalu;
 
   if (!gst_h265_decoder_preprocess_slice (self, &priv->current_slice))