[flang] Fixed uninitialized std::unique_ptr dereference.

This fixes unittest failures reported in D142279:
  flang-Unit :: Frontend/./FlangFrontendTests/5/7
  flang-Unit :: Frontend/./FlangFrontendTests/6/7

(cherry picked from commit e054e0d)

Author: vzakhari

flang/lib/Frontend/FrontendActions.cpp

@@ -779,7 +779,7 @@ void CodeGenAction::executeAction() {
   llvmModule->setDataLayout(tm->createDataLayout());
 
   // Run LLVM's middle-end (i.e. the optimizer).
-  runOptimizationPipeline(*os);
+  runOptimizationPipeline(ci.isOutputStreamNull() ? *os : ci.getOutputStream());
 
   if (action == BackendActionTy::Backend_EmitLL) {
     llvmModule->print(ci.isOutputStreamNull() ? *os : ci.getOutputStream(),