TaskControl may interrupt non-exist threads

**Describe the bug (描述bug)**
TaskControl may interrupt non-exist threads

```
E1201 22:37:01.713448 34144 /home/zyh/brpc/src/bthread/task_control.cpp:167] Fail to create _workers[2], Resource temporarily unavailable
E1201 22:37:01.713607 34144 /home/zyh/brpc/src/bthread/bthread.cpp:92] Fail to init g_task_control
ASAN:DEADLYSIGNAL
=================================================================
==34144==ERROR: AddressSanitizer: SEGV on unknown address 0x7fc5d05ac9d0 (pc 0x7fc5e7ec4386 bp 0x7ffd1e92b4c0 sp 0x7ffd1e92b460 T0)
==34144==The signal is caused by a READ memory access.
E1201 22:37:01.729068 34147 /home/zyh/brpc/src/bthread/task_control.cpp:68] Fail to create TaskGroup in pthread=140487748658944
E1201 22:37:01.729776 34148 /home/zyh/brpc/src/bthread/task_control.cpp:68] Fail to create TaskGroup in pthread=140487664776960
#0 0x7fc5e7ec4385 (/lib/x86_64-linux-gnu/libpthread.so.0+0xf385)
#1 0x55c92471ddd4 (/home/zyh/braft/example/atomic/atomic_test+0x1a4dd4) brpc/src/bthread/task_control.cpp:240
#2 0x55c92472088a (/home/zyh/braft/example/atomic/atomic_test+0x1a788a) brpc/src/bthread/task_control.cpp:257
#3 0x55c92471cb75 (/home/zyh/braft/example/atomic/atomic_test+0x1a3b75) brpc/src/bthread/bthread.cpp:93
#4 0x55c924764451 (/home/zyh/braft/example/atomic/atomic_test+0x1eb451) brpc/src/brpc/global.cpp:607
#5 0x7fc5e7ec4906 (/lib/x86_64-linux-gnu/libpthread.so.0+0xf906)
#6 0x55c92476130c (/home/zyh/braft/example/atomic/atomic_test+0x1e830c)
#7 0x55c92473d280 (/home/zyh/braft/example/atomic/atomic_test+0x1c4280)
#8 0x55c92473db2e (/home/zyh/braft/example/atomic/atomic_test+0x1c4b2e)
#9 0x55c9246d6e45 (/home/zyh/braft/example/atomic/atomic_test+0x15de45)
#10 0x55c9246c5895 (/home/zyh/braft/example/atomic/atomic_test+0x14c895)
#11 0x55c9246b5347 (/home/zyh/braft/example/atomic/atomic_test+0x13c347)
#12 0x7fc5e6f49bf6 (/lib/x86_64-linux-gnu/libc.so.6+0x21bf6)
#13 0x55c9246c3569 (/home/zyh/braft/example/atomic/atomic_test+0x14a569)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib/x86_64-linux-gnu/libpthread.so.0+0xf385)
==34144==ABORTING
```

We can see that the TaskGroup init failed, but the `stop_and_join` function still tries to interrupt all threads which not exist.

**To Reproduce (复现方法)**
Maybe we can add a simple unit test.

**Expected behavior (期望行为)**

Successfully cleanup TaskControl object.

**Versions (各种版本)**
OS: Ubuntu 18.04.6
Compiler: gcc 7
brpc: 0.9.7
protobuf: 3.0.0

**Additional context/screenshots (更多上下文/截图)**

Start in src/bthread/task_control.cpp around the TaskGroup creation failure at line 68, worker creation at line 167, and cleanup paths around lines 240 and 257; compare these with the initialization call in src/bthread/bthread.cpp around lines 92-93. Reproduce the resource-exhaustion failure, then add a focused unit test if practical. Done means TaskControl cleanup succeeds without interrupting threads that were not created.

Written by the indexing model from the issue text.