Fix pointer signedness warning in mysqlnd

Author: nikic

ext/mysqlnd/mysqlnd_commands.c

@@ -411,7 +411,8 @@ MYSQLND_METHOD(mysqlnd_command, stmt_execute)(MYSQLND_CONN_DATA * conn, const MY
 
 	DBG_ENTER("mysqlnd_command::stmt_execute");
 
-	ret = send_command(conn->payload_decoder_factory, COM_STMT_EXECUTE, payload.s, payload.l, FALSE,
+	ret = send_command(conn->payload_decoder_factory, COM_STMT_EXECUTE,
+					   (const unsigned char *) payload.s, payload.l, FALSE,
 					   &conn->state,
 					   conn->error_info,
 					   conn->upsert_status,