latmenc: Fix ALS in LATM.

"Fix" in so far as at least it will no longer overread and possibly
crash and makes somewhat sense, but no idea whether there is anything
that can play the resulting files (FFmpeg can't).
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavformat/latmenc.c

@@ -57,6 +57,11 @@ static int latm_decode_extradata(LATMContext *ctx, uint8_t *buf, int size)
     if (ctx->off < 0)
         return ctx->off;
 
+    if (ctx->object_type == AOT_ALS && (ctx->off & 7)) {
+        // as long as avpriv_mpeg4audio_get_config works correctly this is impossible
+        av_log(ctx, AV_LOG_ERROR, "BUG: ALS offset is not byte-aligned\n");
+        return AVERROR_INVALIDDATA;
+    }
     /* FIXME: are any formats not allowed in LATM? */
 
     if (m4ac.object_type > AOT_SBR && m4ac.object_type != AOT_ALS) {
@@ -106,8 +111,8 @@ static void latm_write_frame_header(AVFormatContext *s, PutBitContext *bs)
 
         /* AudioSpecificConfig */
         if (ctx->object_type == AOT_ALS) {
-            header_size = avctx->extradata_size-(ctx->off + 7) >> 3;
-            avpriv_copy_bits(bs, &avctx->extradata[ctx->off], header_size);
+            header_size = avctx->extradata_size-(ctx->off >> 3);
+            avpriv_copy_bits(bs, &avctx->extradata[ctx->off >> 3], header_size);
         } else {
             avpriv_copy_bits(bs, avctx->extradata, ctx->off + 3);