socketpair: clear 'err' when retrying due to EINTR

If the first write was interrupted by a signal and a subsequent write
succeeds, the function would still erroneously return EINTR.

Found by Codex Security
Closes #20809

lib/socketpair.c

@@ -317,6 +317,7 @@ int Curl_wakeup_signal(curl_socket_t socks[2])
 #endif
 
   while(1) {
+    err = 0;
     if(wakeup_write(socks[1], buf, sizeof(buf)) < 0) {
       err = SOCKERRNO;
 #ifdef USE_WINSOCK