[lldb] followup fix for https://reviews.llvm.org/D62305

[lldb] followup fix for https://reviews.llvm.org/D62305

Summary:
Fixing this error on windows build bot:

```
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(21): error C2440: 'initializing': cannot convert from 'nullptr' to 'lldb::thread_result_t'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(21): note: A native nullptr can only be converted to bool or, using reinterpret_cast, to an integral type
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(21): error C2439: 'lldb_private::HostNativeThreadBase::m_result': member could not be initialized
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\include\lldb/Host/HostNativeThreadBase.h(48): note: see declaration of 'lldb_private::HostNativeThreadBase::m_result'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(24): error C2440: 'initializing': cannot convert from 'nullptr' to 'lldb::thread_result_t'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(24): note: A native nullptr can only be converted to bool or, using reinterpret_cast, to an integral type
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(24): error C2439: 'lldb_private::HostNativeThreadBase::m_result': member could not be initialized
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\include\lldb/Host/HostNativeThreadBase.h(48): note: see declaration of 'lldb_private::HostNativeThreadBase::m_result'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(40): error C2440: '=': cannot convert from 'nullptr' to 'lldb::thread_result_t'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(40): note: A native nullptr can only be converted to bool or, using reinterpret_cast, to an integral type
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(50): error C2440: '=': cannot convert from 'nullptr' to 'lldb::thread_result_t'
E:\build_slave\lldb-x64-windows-ninja\llvm\tools\lldb\source\Host\common\HostNativeThreadBase.cpp(50): note: A native nullptr can only be converted to bool or, using reinterpret_cast, to an integral type
```

see http://lab.llvm.org:8011/builders/lldb-x64-windows-ninja/builds/5050/steps/build/logs/stdio

Reviewers: stella.stamenova, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D62337

llvm-svn: 361565