[vscode] fix ubsan problem in the progress event reporter

The error

UndefinedBehaviorSanitizer: undefined-behavior /Users/buildslave/jenkins/workspace/lldb-cmake-sanitized/llvm-project/lldb/tools/lldb-vscode/ProgressEvent.cpp:89:64 in

was found in https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-sanitized/1910/consoleFull#-15641370498254eaf0-7326-4999-85b0-388101f2d404

It turns out that we were not setting m_event_type when initializatin
and update case. The fix is very simple.

Differential Revision: https://reviews.llvm.org/D105832