Replace post-increment on iterators with pre-increment

This is a micro-optimization, as pre-increment is at least as efficient
as post-increment and typically slightly more so because it doesn't need
to make a copy of the iterator, and better conforms to the prevailing
C++ style.

Closes https://github.com/wxWidgets/wxWidgets/pull/655

src/generic/datavgen.cpp

@@ -3654,7 +3654,7 @@ public:
 
         if( node->GetItem() == *m_iter )
         {
-            m_iter++;
+            ++m_iter;
             return DoJob::CONTINUE;
         }
         else