title: wxFlexGridSizer AddGrowableCol throwing error
body: <p>I have several wxFlexGridSizers that used to work in wxWigets version 2.8.11. After upgrading the wxWidgets library to 2.9.4 I am now getting an invalid column index in wxFlexGridSizer::AddGrowableCol().</p>
  
  <p>Here is a sample of the sizer that I am creating:</p>
  
  <pre><code>wxFlexGridSizer* fgSizer1;
  fgSizer1 = new wxFlexGridSizer( 3, 2, 0, 0 );
  fgSizer1-&gt;AddGrowableCol( 2 );
  fgSizer1-&gt;SetFlexibleDirection( wxBOTH );
  fgSizer1-&gt;SetNonFlexibleGrowMode( wxFLEX_GROWMODE_SPECIFIED );
  </code></pre>
  
  <p>I tried changing the value of the AddGrowableCol() arguement to 1 because maybe I thought it might have been wrong before or something but it still throws the same error. I can include the stacktrace if that would be helpful as well.</p>
  
  <pre><code>..\..\src\common\sizer.cpp(1956): assert "!m_cols || idx &lt; (size_t)m_cols" failed in wxFlexGridSizer::AddGrowableCol(): invalid column index
  
  Call stack:
  [00] wxGUIAppTraitsBase::ShowAssertDialog       j:\wxwidgets-2.9.4\src\common\appcmn.cpp:475
  [01] ShowAssertDialog                           j:\wxwidgets-2.9.4\src\common\appbase.cpp:1265
  [02] wxAppConsoleBase::OnAssertFailure          j:\wxwidgets-2.9.4\src\common\appbase.cpp:761
  [03] wxDefaultAssertHandler                     j:\wxwidgets-2.9.4\src\common\appbase.cpp:1065
  [04] wxOnAssert                                 j:\wxwidgets-2.9.4\src\common\appbase.cpp:1141
  [05] wxFlexGridSizer::AddGrowableCol            j:\wxwidgets-2.9.4\src\common\sizer.cpp:1956
  [06] MyFrame::MyFrame                           c:\users\james\documents\code\wx29starter - copy (2)\minimal.cpp:181
  [07] MyApp::OnInit                              c:\users\james\documents\code\wx29starter - copy (2)\minimal.cpp:130
  [08] wxAppConsoleBase::CallOnInit               j:\wxwidgets-2.9.4\include\wx\app.h:94
  [09] wxEntryReal                                j:\wxwidgets-2.9.4\src\common\init.cpp:456
  [10] wxEntry                                    j:\wxwidgets-2.9.4\src\msw\main.cpp:189
  [11] wxEntry                                    j:\wxwidgets-2.9.4\src\msw\main.cpp:416
  [12] WinMain                                    c:\users\james\documents\code\wx29starter - copy (2)\minimal.cpp:111
  [13] __tmainCRTStartup                          f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c:578
  [14] WinMainCRTStartup                          f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c:403
  [15] BaseThreadInitThunk                     
  [16] RtlInitializeExceptionChain             
  [17] RtlInitializeExceptionChain         
  </code></pre>
  
  <p>Here is the assert code</p>
  
  <pre><code>void wxFlexGridSizer::AddGrowableCol( size_t idx, int proportion )
  {
      wxASSERT_MSG( !IsColGrowable( idx ),
                    "AddGrowableCol() called for growable column" );
  
      // see comment in AddGrowableRow(): although it's less common to omit the
      // specification of the number of columns, it still can also happen
      wxCHECK_RET( !m_cols || idx &lt; (size_t)m_cols, "invalid column index" );
  
      m_growableCols.Add( idx );
      m_growableColsProportions.Add( proportion );
  }
  </code></pre>