PVStructure::getStructure() avoid creation of temporary

avoid some ref-counter activity

src/factory/PVStructure.cpp

@@ -74,7 +74,7 @@ void PVStructure::setImmutable()
     PVField::setImmutable();
 }
 
-StructureConstPtr PVStructure::getStructure() const
+const StructureConstPtr& PVStructure::getStructure() const
 {
     return structurePtr;
 }

src/pv/pvData.h

@@ -756,7 +756,7 @@ public:
      * Get the introspection interface
      * @return The interface.
      */
-    StructureConstPtr getStructure() const;
+    const StructureConstPtr &getStructure() const;
     /**
      * Get the array of pointers to the subfields in the structure.
      * @return The array.