lavc/hevc: Don't parse NAL unit for a dummy buffer

hevc parser mistakenly reports the following message if a dummy buffer
is padded for EOF

   [hevc @ 0x559b63848610] missing picture in access unit

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>
Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Reviewed-by: "Li, Zhong" <zhong.li@intel.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevc_parser.c

@@ -294,6 +294,8 @@ static int hevc_parse(AVCodecParserContext *s, AVCodecContext *avctx,
     int next;
     HEVCParserContext *ctx = s->priv_data;
     ParseContext *pc = &ctx->pc;
+    int is_dummy_buf = !buf_size;
+    const uint8_t *dummy_buf = buf;
 
     if (avctx->extradata && !ctx->parsed_extradata) {
         ff_hevc_decode_extradata(avctx->extradata, avctx->extradata_size, &ctx->ps, &ctx->sei,
@@ -313,7 +315,10 @@ static int hevc_parse(AVCodecParserContext *s, AVCodecContext *avctx,
         }
     }
 
-    parse_nal_units(s, buf, buf_size, avctx);
+    is_dummy_buf = (is_dummy_buf && (dummy_buf == buf));
+
+    if (!is_dummy_buf)
+        parse_nal_units(s, buf, buf_size, avctx);
 
     *poutbuf      = buf;
     *poutbuf_size = buf_size;