Curl_expire_latest: ignore already expired timers

If the existing timer is still in there but has expired, the new timer
should be added.

Reported-by: Rainer Canavan
Bug: https://curl.haxx.se/mail/lib-2017-04/0030.html
Closes #1407

lib/multi.c

@@ -3003,10 +3003,13 @@ void Curl_expire_latest(struct Curl_easy *data, time_t milli)
        Compare if the new time is earlier, and only remove-old/add-new if it
        is. */
     time_t diff = curlx_tvdiff(set, *expire);
-    if(diff > 0)
-      /* the new expire time was later than the top time, so just skip this */
+    if((diff > 0) && (diff < milli)) {
+      /* if the new expire time is later than the top time, skip it, but not
+         if the diff is larger than the new offset since then the previous
+         time is already expired! */
       return;
     }
+  }
 
   /* Just add the timeout like normal */
   Curl_expire(data, milli);

tests/data/test1238

@@ -10,7 +10,7 @@ TFTP RRQ
 # Server-side
 <reply>
 <servercmd>
-writedelay: 1
+writedelay: 2
 </servercmd>
 # ~1200 bytes (so that they don't fit in two 512 byte chunks)
 <data nocheck="yes">