Curl_socket_check: enlarge poll struct array to 3

This function was introduced in commit 5527417afa and as pointed out
by Gokhan Sengun, the array with poll structs must large enough to hold
3 sockets since that is what the function can accept. It could be noted
that he had this fixed in his patch as posted in
http://curl.haxx.se/mail/lib-2011-12/0179.html

Bug: http://curl.haxx.se/mail/lib-2011-12/0228.html
Reported by: Gokhan Sengun

lib/select.c

@@ -153,7 +153,7 @@ int Curl_socket_check(curl_socket_t readfd0, /* two sockets to read from */
                       long timeout_ms)       /* milliseconds to wait */
 {
 #ifdef HAVE_POLL_FINE
-  struct pollfd pfd[2];
+  struct pollfd pfd[3];
   int num;
 #else
   struct timeval pending_tv;