Fix segfault in SpeedTestClient::mkSocket (taganaka#14)

Replace the use of the non-reentrant gethostbyname() function by the
thread-safe gethostbyname_r() one. Those segfaults are more likely
to happen with a high number of threads, which is the case when testing
high-bandwidth connections.

This is also dependent on the libc/compiler, as the program started to
segfault ~100% of the time when attempting to test a Fiber connection
with the program compiled under gcc7 where the same program compiled
under gcc5 was segfaulting ~%5 of the time.

SpeedTestClient.cpp

@@ -188,17 +188,19 @@ bool SpeedTestClient::mkSocket() {
     }
 
     auto hostp = hostport();
-
-    struct hostent *server = gethostbyname(hostp.first.c_str());
-    if (server == nullptr) {
+    struct hostent server;
+    char tmpbuf[BUFSIZ];
+    struct hostent *result;
+    int errnop;
+    if (gethostbyname_r(hostp.first.c_str(), &server, (char *)&tmpbuf, BUFSIZ, &result, &errnop)) {
         return false;
     }
 
     int portno = hostp.second;
     struct sockaddr_in serv_addr{};
     memset(&serv_addr, 0, sizeof(serv_addr));
     serv_addr.sin_family = AF_INET;
-    memcpy(&serv_addr.sin_addr.s_addr, server->h_addr, (size_t)server->h_length);
+    memcpy(&serv_addr.sin_addr.s_addr, server.h_addr, (size_t)server.h_length);
 
     serv_addr.sin_port = htons(static_cast<uint16_t>(portno));