[lldb] Fix Process::SyncIOHandler

D157648 broke the function because it put the blocking wait into a
critical section. This meant that, if m_iohandler_sync was not updated
before entering the function, no amount of waiting would help.

Fix that by restriciting the scope of the critical section to the
iohandler check.

Author: labath

lldb/source/Target/Process.cpp

@@ -627,8 +627,7 @@ void Process::SyncIOHandler(uint32_t iohandler_id,
                             const Timeout<std::micro> &timeout) {
   // don't sync (potentially context switch) in case where there is no process
   // IO
-  std::lock_guard<std::mutex> guard(m_process_input_reader_mutex);
-  if (!m_process_input_reader)
+  if (!ProcessIOHandlerExists())
     return;
 
   auto Result = m_iohandler_sync.WaitForValueNotEqualTo(iohandler_id, timeout);