修复空指针导致进程产生core dump

[az13js]

4.2.0版本，在本地编译安装后，在php-fpm日志发现每个请求进来后都没能返回完整的响应，查日志发现 core dump 。

[13-Feb-2022 17:17:10.329551] DEBUG: pid 1024955, fpm_got_signal(), line 82: received SIGCHLD
[13-Feb-2022 17:17:10.329577] DEBUG: pid 1024955, fpm_event_loop(), line 430: event module triggered 1 events
[13-Feb-2022 17:17:10.329602] WARNING: pid 1024955, fpm_children_bury(), line 259: [pool fpms4] child 1024964 exited on signal 11 (SIGSEGV - core dumped) after 50.634819 seconds from start
[13-Feb-2022 17:17:10.329608] DEBUG: pid 1024955, fpm_children_make(), line 407: blocking signals before child birth
[13-Feb-2022 17:17:10.329886] DEBUG: pid 1024955, fpm_children_make(), line 431: unblocking signals, child born
[13-Feb-2022 17:17:10.329929] NOTICE: pid 1024955, fpm_children_make(), line 437: [pool fpms4] child 1025215 started

用GDB调试打开 core dump 文件发现：

(gdb) bt
#0  0x00007f1ec3ee97f8 in Segment::skip() ()
   from /opt/php7.4.16/lib/php/extensions/no-debug-non-zts-20190902/skywalking.so
#1  0x00007f1ec3ef0292 in sky_request_flush(_zval_struct*, unsigned long) ()
   from /opt/php7.4.16/lib/php/extensions/no-debug-non-zts-20190902/skywalking.so
#2  0x00007f1ec3ea6402 in zm_deactivate_skywalking ()
   from /opt/php7.4.16/lib/php/extensions/no-debug-non-zts-20190902/skywalking.so
#3  0x000055c3a9743c41 in zend_deactivate_modules ()
#4  0x000055c3a966c0d7 in php_request_shutdown ()
#5  0x000055c3a98f7281 in main ()
(gdb) 

调试代码，发现是空指针：

// 函数 sky_request_flush：
auto *segment = sky_get_segment(nullptr, request_id);
if (segment->skip()) {
    delete segment;
    sky_remove_segment(request_id);
    
    return;
}

我加了个判断，如果 segment 是空指针的话就直接返回了。

[az13js]

4.2.0版本，在本地编译安装后，在php-fpm日志发现每个请求进来后都没能返回完整的响应，查日志发现 core dump 。

4.2.0版本，在本地编译安装后，发现每个请求进来后都没能返回完整的响应，查日志发现 core dump 。

[heyanlong]

LGTM