message: Set m_struct_valid to initial value in ctor.
  Patch from Tom Tromey
diff: @@ -48,7 +48,8 @@ FunctionCaller::FunctionCaller(ExecutionContextScope &exe_scope,
  m_function_return_type(return_type),
  m_wrapper_function_name("__lldb_caller_function"),
  m_wrapper_struct_name("__lldb_caller_struct"), m_wrapper_args_addrs(),
  - m_arg_values(arg_value_list), m_compiled(false), m_JITted(false) {
  + m_struct_valid(false), m_arg_values(arg_value_list), m_compiled(false),
  + m_JITted(false) {
  m_jit_process_wp = lldb::ProcessWP(exe_scope.CalculateProcess());
  // Can't make a FunctionCaller without a process.
  assert(m_jit_process_wp.lock());
  