Fix expressions leaking internal data

(cherry-picked from 15dcb6)

Author: nyalldawson

src/core/qgsexpression.cpp

@@ -3697,6 +3697,11 @@ QgsExpression::QgsExpression( const QgsExpression& other )
 
 QgsExpression& QgsExpression::operator=( const QgsExpression & other )
 {
+  if ( !d->ref.deref() )
+  {
+    delete d;
+  }
+
   d = other.d;
   d->ref.ref();
   return *this;