ff_mss12_decode_init: check dimensions

Fixes assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mss12.c

@@ -586,6 +586,11 @@ av_cold int ff_mss12_decode_init(MSS12Context *c, int version,
                avctx->coded_width, avctx->coded_height);
         return AVERROR_INVALIDDATA;
     }
+    if (avctx->coded_width < 1 || avctx->coded_height < 1) {
+        av_log(avctx, AV_LOG_ERROR, "Frame dimensions %dx%d too small",
+               avctx->coded_width, avctx->coded_height);
+        return AVERROR_INVALIDDATA;
+    }
 
     av_log(avctx, AV_LOG_DEBUG, "Encoder version %d.%d\n",
            AV_RB32(avctx->extradata + 4), AV_RB32(avctx->extradata + 8));