[flang] Add explicit nullptr check in initialisation of inDoConstruct.

This explicit check is needed as we are using braced initialisation here so
implicit narrowing conversions (such as pointer to bool) are not allowed.

Original-commit: flang-compiler/f18@ff8dad10bf
Reviewed-on: https://github.com/flang-compiler/f18/pull/1083

flang/lib/Semantics/check-do-forall.cpp

@@ -912,7 +912,7 @@ void DoForallChecker::CheckForBadLeave(
 static bool StmtMatchesConstruct(const parser::Name *stmtName,
     StmtType stmtType, const parser::Name *constructName,
     const ConstructNode &construct) {
-  bool inDoConstruct{MaybeGetDoConstruct(construct)};
+  bool inDoConstruct{MaybeGetDoConstruct(construct) != nullptr};
   if (!stmtName) {
     return inDoConstruct;  // Unlabeled statements match all DO constructs
   } else if (constructName && constructName->source == stmtName->source) {