Segmentation fault: pthread_mutex_lock_imp

**Describe the bug (描述bug)**

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff58e9850 in pthread_mutex_lock_impl (mutex=0x8097f0) at ../source/brpc/src/bthread/mutex.cpp:554
554 return sys_pthread_mutex_lock(mutex)

**To Reproduce (复现方法)**
我在程序中，启动运行即出现，coredump见附件

**Expected behavior (期望行为)**

**Versions (各种版本)**
OS: CentOS 7.6 3.10.0-957.el7.x86_64
Compiler: 7.3.1
brpc: tag 1.0.0
braft: tag 1.2.0
protobuf: protobuf-2.5.0-8.el7.x86_64 protobuf-compiler-2.5.0-8.el7.x86_64

**Additional context/screenshots (更多上下文/截图)**
[core_petrelfs-mgmtd_248630_1646794226.zip](https://github.com/apache/incubator-brpc/files/8211465/core_petrelfs-mgmtd_248630_1646794226.zip)

Start with the attached core dump and bthread/mutex.cpp:554, then inspect the reported pthread_mutex_lock_impl crash in the brpc 1.0.0 and braft 1.2.0 environment. Done means identifying a reproducible cause and verifying a resolution or clear explanation of the failure.

Written by the indexing model from the issue text.