avcodec/fic: fix slice checks

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

libavcodec/fic.c

@@ -214,8 +214,8 @@ static int fic_decode_frame(AVCodecContext *avctx, void *data,
     }
 
     for (slice = 0; slice < nslices; slice++) {
-        int slice_off = AV_RB32(src + tsize + FIC_HEADER_SIZE + slice * 4);
-        int slice_size;
+        unsigned slice_off = AV_RB32(src + tsize + FIC_HEADER_SIZE + slice * 4);
+        unsigned slice_size;
         int y_off   = ctx->slice_h * slice;
         int slice_h = ctx->slice_h;
 
@@ -230,11 +230,11 @@ static int fic_decode_frame(AVCodecContext *avctx, void *data,
             slice_size = AV_RB32(src + tsize + FIC_HEADER_SIZE + slice * 4 + 4);
         }
 
-        slice_size -= slice_off;
-
-        if (slice_off > msize || slice_off + slice_size > msize)
+        if (slice_size < slice_off || slice_size > msize)
             continue;
 
+        slice_size -= slice_off;
+
         ctx->slice_data[slice].src      = sdata + slice_off;
         ctx->slice_data[slice].src_size = slice_size;
         ctx->slice_data[slice].slice_h  = slice_h;