memory for alpha channel was over allocated by a factor of 3

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@20569 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/image.cpp

@@ -809,8 +809,7 @@ void wxImage::SetAlpha( unsigned char *alpha )
 
     if ( !alpha )
     {
-        alpha = (unsigned char *)
-                    malloc(M_IMGDATA->m_width*M_IMGDATA->m_height*3);
+        alpha = (unsigned char *)malloc(M_IMGDATA->m_width*M_IMGDATA->m_height);
     }
 
     delete [] M_IMGDATA->m_alpha;