avcodec/flashsv: Check size before updating it

Fixes out of array read
Fixes: 3c857d4d90365731524716e6d051e43a/signal_sigsegv_7f4f59bcc29e_1386_20abd2c8e655cb9c75b24368e65fe3b1.flv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/flashsv.c

@@ -413,6 +413,10 @@ static int flashsv_decode_frame(AVCodecContext *avctx, void *data,
                 }
 
                 if (has_diff) {
+                    if (size < 3) {
+                        av_log(avctx, AV_LOG_ERROR, "size too small for diff\n");
+                        return AVERROR_INVALIDDATA;
+                    }
                     if (!s->keyframe) {
                         av_log(avctx, AV_LOG_ERROR,
                                "Inter frame without keyframe\n");
@@ -440,6 +444,10 @@ static int flashsv_decode_frame(AVCodecContext *avctx, void *data,
                     int row = get_bits(&gb, 8);
                     av_log(avctx, AV_LOG_DEBUG, "%dx%d zlibprime_curr %dx%d\n",
                            i, j, col, row);
+                    if (size < 3) {
+                        av_log(avctx, AV_LOG_ERROR, "size too small for zlibprime_curr\n");
+                        return AVERROR_INVALIDDATA;
+                    }
                     size -= 2;
                     avpriv_request_sample(avctx, "zlibprime_curr");
                     return AVERROR_PATCHWELCOME;