protect libedit and LLDB gui from receiving null FILE* streams

[lawrence_danna]

Summary

We now have valid files that will return NULL from GetStream().
libedit and the LLDB gui are the only places left that need FILE*
streams. Both are doing curses-like user interaction that only
make sense with a real terminal anyway, so there is no need to convert
them off of their use of FILE*. But we should check for null streams
before enabling these features.

[lawrence_danna]

rebased

lldb/source/Commands/CommandObjectGUI.cpp

 CommandObjectGUI::~CommandObjectGUI() {}
 
 bool CommandObjectGUI::DoExecute(Args &args, CommandReturnObject &result) {
 #ifndef LLDB_DISABLE_CURSES
   if (args.GetArgumentCount() == 0) {
     Debugger &debugger = GetDebugger();
 
     File &input = debugger.GetInputFile();
-    if (input.GetIsRealTerminal() && input.GetIsInteractive()) {
+    File &output = debugger.GetOutputFile();
+    if (input.GetStream() && output.GetStream() && input.GetIsRealTerminal() &&
+        input.GetIsInteractive()) {
       IOHandlerSP io_handler_sp(new IOHandlerCursesGUI(debugger));
       if (io_handler_sp)
         debugger.PushIOHandler(io_handler_sp);
       result.SetStatus(eReturnStatusSuccessFinishResult);
     } else {
       result.AppendError("the gui command requires an interactive terminal.");
       result.SetStatus(eReturnStatusFailed);
     }

lldb/source/Core/IOHandler.cpp

#endif
       m_curr_line_idx(UINT32_MAX), m_multi_line(multi_line),
       m_color_prompts(color_prompts), m_interrupt_exits(true),
       m_editing(false) {
   SetPrompt(prompt);
 
 #ifndef LLDB_DISABLE_LIBEDIT
   bool use_editline = false;
 
-  use_editline = m_input_sp && m_input_sp->GetIsRealTerminal();
+  use_editline = GetInputFILE() && GetOutputFILE() && GetErrorFILE() &&
+                 m_input_sp && m_input_sp->GetIsRealTerminal();
 
   if (use_editline) {
     m_editline_up.reset(new Editline(editline_name, GetInputFILE(),
                                      GetOutputFILE(), GetErrorFILE(),
                                      m_color_prompts));
     m_editline_up->SetIsInputCompleteCallback(IsInputCompleteCallback, this);
     m_editline_up->SetAutoCompleteCallback(AutoCompleteCallback, this);
     // See if the delegate supports fixing indentation