Don't leak the MYSQL connection handle if there was an exception earlier

The function `disconnect_and_raise` set connected = 0, which prevented
freeing the MYSQL connection handle. Switch to simply checking whether
the pointer to MYSQL handle is NULL or not before freeing it. Thanks to
Chris Bandy for noticing this problem while working on Client#close.

Author: sodabrew

ext/mysql2/client.c

@@ -209,12 +209,13 @@ static VALUE invalidate_fd(int clientfd)
 #endif /* _WIN32 */
 
 static void *nogvl_close(void *ptr) {
-  mysql_client_wrapper *wrapper;
-  wrapper = ptr;
-  if (wrapper->connected) {
-    MARK_CONN_INACTIVE(self);
-    wrapper->connected = 0;
+  mysql_client_wrapper *wrapper = ptr;
+
+  if (wrapper->client) {
     mysql_close(wrapper->client);
+    wrapper->client = NULL;
+    wrapper->connected = 0;
+    wrapper->active_thread = Qnil;
   }
 
   return NULL;