indeo3: initialise pixel planes on allocation

This prevents decoder from reading garbage from it in case of errors later.
(cherry picked from commit 81064a8045)

Fixes an invalid read on sample from CVE-2012-2804

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/indeo3.c

@@ -194,6 +194,8 @@ static av_cold int allocate_frame_buffers(Indeo3DecodeContext *ctx,
         /* set buffer pointers = buf_ptr + pitch and thus skip the INTRA prediction line */
         ctx->planes[p].pixels[0] = ctx->planes[p].buffers[0] + ctx->planes[p].pitch;
         ctx->planes[p].pixels[1] = ctx->planes[p].buffers[1] + ctx->planes[p].pitch;
+        memset(ctx->planes[p].pixels[0], 0, ctx->planes[p].pitch * ctx->planes[p].height);
+        memset(ctx->planes[p].pixels[1], 0, ctx->planes[p].pitch * ctx->planes[p].height);
     }
 
     return 0;