avfilter/vf_pullup: fix gray8

Fixes segfault
Fixes Ticket3469

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_pullup.c

@@ -256,6 +256,8 @@ static int alloc_buffer(PullupContext *s, PullupBuffer *b)
     for (i = 0; i < s->nb_planes; i++) {
         b->planes[i] = av_malloc(s->planeheight[i] * s->planewidth[i]);
     }
+    if (s->nb_planes == 1)
+        b->planes[1] = av_malloc(4*256);
 
     return 0;
 }