lavc/encode: fix frame_number double-counted

Encoder frame_number may be double-counted if some frames are cached and then flushed.
Take qsv encoder (some frames are cached firsty for asynchronism) as example,
./ffmpeg -loglevel verbose -hwaccel qsv -c:v h264_qsv -i in.mp4 -vframes 100 -c:v h264_qsv out.mp4
frame_number passed to encoder is double-counted and larger than the accurate value.
Libx264 encoding with B frames can also reproduce it.

Signed-off-by: Zhong Li <zhong.li@intel.com>

libavcodec/encode.c

@@ -234,8 +234,8 @@ int attribute_align_arg avcodec_encode_audio2(AVCodecContext *avctx,
             if (ret >= 0)
                 avpkt->data = avpkt->buf->data;
         }
-
-        avctx->frame_number++;
+        if (frame)
+            avctx->frame_number++;
     }
 
     if (ret < 0 || !*got_packet_ptr) {
@@ -329,7 +329,8 @@ int attribute_align_arg avcodec_encode_video2(AVCodecContext *avctx,
                 avpkt->data = avpkt->buf->data;
         }
 
-        avctx->frame_number++;
+        if (frame)
+            avctx->frame_number++;
     }
 
     if (ret < 0 || !*got_packet_ptr)