libavcodec/webp: treat out-of-bound palette index as translucent black

See https://code.google.com/p/webp/issues/detail?id=206
for a description of the problem/fix.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

This patch makes the decoder follow the recommendation of the spec.
There is some disagreement (see "[FFmpeg-devel] [PATCH]: libavcodec/webp")
about what would be best to be written in the spec, so in case the spec
is changed again, this potentially would need to be amended or reverted

libavcodec/webp.c

@@ -1028,7 +1028,7 @@ static int apply_color_indexing_transform(WebPContext *s)
     ImageContext *img;
     ImageContext *pal;
     int i, x, y;
-    uint8_t *p, *pi;
+    uint8_t *p;
 
     img = &s->image[IMAGE_ROLE_ARGB];
     pal = &s->image[IMAGE_ROLE_COLOR_INDEXING];
@@ -1066,11 +1066,11 @@ static int apply_color_indexing_transform(WebPContext *s)
             p = GET_PIXEL(img->frame, x, y);
             i = p[2];
             if (i >= pal->frame->width) {
-                av_log(s->avctx, AV_LOG_ERROR, "invalid palette index %d\n", i);
-                return AVERROR_INVALIDDATA;
+                AV_WB32(p, 0xFF000000);
+            } else {
+                const uint8_t *pi = GET_PIXEL(pal->frame, i, 0);
+                AV_COPY32(p, pi);
             }
-            pi = GET_PIXEL(pal->frame, i, 0);
-            AV_COPY32(p, pi);
         }
     }