lavc: check channel count after decoder init

Ensures the decoder did not set channel count to an insanely high value
during initialization, which could cause large memory usage when it tries to
get a buffer during decoding.

libavcodec/utils.c

@@ -881,6 +881,11 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
                 avctx->channel_layout = 0;
             }
         }
+        if (avctx->channels && avctx->channels < 0 ||
+            avctx->channels > FF_SANE_NB_CHANNELS) {
+            ret = AVERROR(EINVAL);
+            goto free_and_end;
+        }
     }
 end:
     entangled_thread_counter--;