tftp: reject file name lengths that don't fit

... and thereby avoid telling send() to send off more bytes than the
size of the buffer!

CVE-2017-1000100

Bug: https://curl.haxx.se/docs/adv_20170809B.html
Reported-by: Even Rouault

Credit to OSS-Fuzz for the discovery

lib/tftp.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
@@ -491,6 +491,11 @@ static CURLcode tftp_send_first(tftp_state_data_t *state, tftp_event_t event)
     if(result)
       return result;
 
+    if(strlen(filename) > (state->blksize - strlen(mode) - 4)) {
+      failf(data, "TFTP file name too long\n");
+      return CURLE_TFTP_ILLEGAL; /* too long file name field */
+    }
+
     snprintf((char *)state->spacket.data+2,
              state->blksize,
              "%s%c%s%c", filename, '\0',  mode, '\0');