Fixed an error in Debugger::UpdateExecutionContext() where an invalid index ID 0

was used to set the selected thread if none was selected.  Use a more robust
API to accomplish the task.

Also fixed an error found, while investigating, in CommandObjectThreadSelect::
Execute() where the return status was not properly set if successful.

As a result, both the stl step-in test cases with expectedFailure decorators now
passed.

llvm-svn: 113825