sanm: Use the correct height variable in the decoded_size checks

Fixes integer overflow and out of array accesses

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

libavcodec/sanm.c

@@ -420,8 +420,8 @@ static int old_codec37(SANMVideoContext *ctx, int top,
     flags        = bytestream2_get_byte(&ctx->gb);
     bytestream2_skip(&ctx->gb, 3);
 
-    if (decoded_size > height * stride - left - top * stride) {
-        decoded_size = height * stride - left - top * stride;
+    if (decoded_size > ctx->height * stride - left - top * stride) {
+        decoded_size = ctx->height * stride - left - top * stride;
         av_log(ctx->avctx, AV_LOG_WARNING, "decoded size is too large\n");
     }
 
@@ -658,8 +658,8 @@ static int old_codec47(SANMVideoContext *ctx, int top,
     decoded_size = bytestream2_get_le32(&ctx->gb);
     bytestream2_skip(&ctx->gb, 8);
 
-    if (decoded_size > height * stride - left - top * stride) {
-        decoded_size = height * stride - left - top * stride;
+    if (decoded_size > ctx->height * stride - left - top * stride) {
+        decoded_size = ctx->height * stride - left - top * stride;
         av_log(ctx->avctx, AV_LOG_WARNING, "decoded size is too large\n");
     }