avcodec/aacdec: Fix runtime error: signed integer overflow: 2147483520 + 255 cannot be represented in type 'int'

Fixes: 1656/clusterfuzz-testcase-minimized-5900404925661184

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aacdec.c

@@ -431,6 +431,8 @@ static int read_payload_length_info(struct LATMContext *ctx, GetBitContext *gb)
     if (ctx->frame_length_type == 0) {
         int mux_slot_length = 0;
         do {
+            if (get_bits_left(gb) < 8)
+                return AVERROR_INVALIDDATA;
             tmp = get_bits(gb, 8);
             mux_slot_length += tmp;
         } while (tmp == 255);
@@ -460,7 +462,7 @@ static int read_audio_mux_element(struct LATMContext *latmctx,
     }
     if (latmctx->audio_mux_version_A == 0) {
         int mux_slot_length_bytes = read_payload_length_info(latmctx, gb);
-        if (mux_slot_length_bytes * 8 > get_bits_left(gb)) {
+        if (mux_slot_length_bytes < 0 || mux_slot_length_bytes * 8LL > get_bits_left(gb)) {
             av_log(latmctx->aac_ctx.avctx, AV_LOG_ERROR, "incomplete frame\n");
             return AVERROR_INVALIDDATA;
         } else if (mux_slot_length_bytes * 8 + 256 < get_bits_left(gb)) {