avformat/hlsenc: improve hls encrypt get key file operation

get key file only once time is ok, no need more times.
Ticket-id: #6545

Found-by: JohnPi
Signed-off-by: Steven Liu <lq@onvideo.cn>

libavformat/hlsenc.c

@@ -1223,6 +1223,8 @@ static int hls_start(AVFormatContext *s)
             av_log(s, AV_LOG_WARNING, "Cannot use both -hls_key_info_file and -hls_enc,"
                   " will use -hls_key_info_file priority\n");
         }
+
+        if (c->number <= 1) {
             if (c->key_info_file) {
                 if ((err = hls_encryption_start(s)) < 0)
                     goto fail;
@@ -1230,6 +1232,7 @@ static int hls_start(AVFormatContext *s)
                 if ((err = do_encrypt(s)) < 0)
                     goto fail;
             }
+        }
         if ((err = av_dict_set(&options, "encryption_key", c->key_string, 0))
                 < 0)
             goto fail;