[lldb-vscode] Don't try to launch an invalid program.

[jgorbe]

Summary

If an invalid program is specified, lldb-vscode will send back a
response with "success": false, but then will continue executing the
rest of request_launch(), try to launch the program anyway and try to
send another response (possibly using the response object after it was
moved).

This change adds a return statement so we stop executing the handler
after producing the first failing response.

tools/lldb-vscode/lldb-vscode.cpp

lldb::SBModule module = g_vsc.target.AddModule(
         program.data(), target_triple, uuid_cstr, symfile);
     if (!module.IsValid()) {
       response["success"] = llvm::json::Value(false);
 
       EmplaceSafeString(
           response, "message",
           llvm::formatv("Could not load program '{0}'.", program).str());
       g_vsc.SendJSON(llvm::json::Value(std::move(response)));
+      return;
     }
   }
 
   // Extract any extra arguments and append them to our program arguments for
   // when we launch
   auto args = GetStrings(arguments, "args");
   if (!args.empty())
     g_vsc.launch_info.SetArguments(MakeArgv(args).data(), true);