file: use private buffer for C-L output

... instead of clobbering the download buffer.

lib/file.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2016, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -475,9 +475,10 @@ static CURLcode file_do(struct connectdata *conn, bool *done)
     time_t filetime;
     struct tm buffer;
     const struct tm *tm = &buffer;
-    snprintf(buf, CURL_BUFSIZE(data->set.buffer_size),
+    char header[80];
+    snprintf(header, sizeof(header),
              "Content-Length: %" CURL_FORMAT_CURL_OFF_T "\r\n", expected_size);
-    result = Curl_client_write(conn, CLIENTWRITE_BOTH, buf, 0);
+    result = Curl_client_write(conn, CLIENTWRITE_BOTH, header, 0);
     if(result)
       return result;
 
@@ -492,7 +493,7 @@ static CURLcode file_do(struct connectdata *conn, bool *done)
       return result;
 
     /* format: "Tue, 15 Nov 1994 12:45:26 GMT" */
-    snprintf(buf, BUFSIZE-1,
+    snprintf(header, sizeof(header),
              "Last-Modified: %s, %02d %s %4d %02d:%02d:%02d GMT\r\n",
              Curl_wkday[tm->tm_wday?tm->tm_wday-1:6],
              tm->tm_mday,