h264-mt: fix deadlock in packets with multiple slices (e.g. MP4).

libavcodec/h264.c

@@ -3685,6 +3685,8 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             switch (hx->nal_unit_type) {
                 case NAL_SPS:
                 case NAL_PPS:
+                case NAL_IDR_SLICE:
+                case NAL_SLICE:
                     nals_needed = nal_index;
             }
             continue;