proresdec_lgpl: explicitly check coff[3] against slice_data_size

The implicit checks via v_data_size and a_data_size don't work in the case
'(hdr_size > 7) && !ctx->alpha_info'.

This fixes segmentation faults due to invalid reads.

This problem was introduced in commit
547c2f00.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/proresdec_lgpl.c

@@ -625,7 +625,7 @@ static int decode_slice(AVCodecContext *avctx, void *tdata)
 
     /* if V or alpha component size is negative that means that previous
        component sizes are too large */
-    if (v_data_size < 0 || a_data_size < 0 || hdr_size < 6) {
+    if (v_data_size < 0 || a_data_size < 0 || hdr_size < 6 || coff[3] > slice_data_size) {
         av_log(avctx, AV_LOG_ERROR, "invalid data size\n");
         return AVERROR_INVALIDDATA;
     }