QtConcurrent: Avoid an allocation in ExceptionHolder if there is no exception to store.

Qt3D hits this several times per frame.

Change-Id: Iaadcfbe79f146bd73b36f01325580dc24a10225c
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/thread/qexception.cpp

@@ -165,7 +165,7 @@ public:
 };
 
 ExceptionHolder::ExceptionHolder(QException *exception)
-: base(new Base(exception)) {}
+: base(exception ? new Base(exception) : Q_NULLPTR) {}
 
 ExceptionHolder::ExceptionHolder(const ExceptionHolder &other)
 : base(other.base)
@@ -181,6 +181,8 @@ ExceptionHolder::~ExceptionHolder()
 
 QException *ExceptionHolder::exception() const
 {
+    if (!base)
+        return Q_NULLPTR;
     return base->exception;
 }