Fix empty wxBitmapComboBox height under wxMSW.

Make (initially) empty wxBitmapComboBox as high as a normal wxComboBox instead
of making it visibly shorter.

Closes #14279.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77548 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/bmpcbox.cpp

@@ -449,7 +449,12 @@ bool wxBitmapComboBox::MSWOnMeasure(WXMEASUREITEMSTRUCT *item)
     LPMEASUREITEMSTRUCT lpMeasureItem = (LPMEASUREITEMSTRUCT) item;
     int pos = lpMeasureItem->itemID;
 
+    // Measure item height if item list is not empty,
+    // otherwise leave default system value.
+    if ( pos >= 0 )
+    {
         lpMeasureItem->itemHeight = wxBitmapComboBoxBase::MeasureItem(pos);
+    }
 
     return TRUE;
 }