vf_transpose: avoid multiple calls to avfilter_draw_slice()

avfilter_draw_slice() is already called in the end_frame() callback,
this avoids multiple calls. This is done by adding a null draw_slice()
callback.

In particular fix crash occurring with -vf transpose=3,hflip, fix trac
issue #371.

libavfilter/vf_transpose.c

@@ -195,6 +195,8 @@ static void end_frame(AVFilterLink *inlink)
     avfilter_unref_buffer(outpic);
 }
 
+static void null_draw_slice(AVFilterLink *link, int y, int h, int slice_dir) { }
+
 AVFilter avfilter_vf_transpose = {
     .name      = "transpose",
     .description = NULL_IF_CONFIG_SMALL("Transpose input video."),
@@ -207,6 +209,7 @@ AVFilter avfilter_vf_transpose = {
     .inputs    = (AVFilterPad[]) {{ .name            = "default",
                                     .type            = AVMEDIA_TYPE_VIDEO,
                                     .start_frame     = start_frame,
+                                    .draw_slice      = null_draw_slice,
                                     .end_frame       = end_frame,
                                     .min_perms       = AV_PERM_READ, },
                                   { .name = NULL}},