Added volatile into a variable, based on advice from andyhhp from SRCF.

[22:40] <andyhhp> curl_process_msgs()'s use of "static int slept" is dangerous and racy.  an optimising compiler can and probably will do bad things
[22:45] <ff266> with respect to "static int slept", should i just put a volatile in front of it? So "volatile static int slept"?
[22:46] <ff266> I meant "static volatile int slept;"
[22:47] <andyhhp> lets say yes for the sake of argument
[22:47] <andyhhp> "its complicated"
[22:47] <andyhhp> but that will broadly do what you want

src/network.c

@@ -107,7 +107,7 @@ static void curl_process_msgs(CURLMsg *curl_msg, int n_running_curl,
 {
     (void) n_running_curl;
     (void) n_mesgs;
-    static int slept = 0;
+    static volatile int slept = 0;
     if (curl_msg->msg == CURLMSG_DONE) {
         TransferStruct *transfer;
         CURL *curl = curl_msg->easy_handle;