Do not try to create bitmap for default constructed Caret

If a Caret is default-constructed, the width and height are still 0,
and the wxBitmap::Create implementations expect valid sizes.

The same applies for the bitmap after a size change.

src/generic/caret.cpp

@@ -103,6 +103,7 @@ void wxCaret::InitGeneric()
 #ifndef wxHAS_CARET_USING_OVERLAYS
     m_xOld =
     m_yOld = -1;
+    if (m_width && m_height)
         m_bmpUnderCaret.Create(m_width, m_height);
 #endif
 }
@@ -174,7 +175,10 @@ void wxCaret::DoSize()
     m_overlay.Reset();
 #else
     // Change bitmap size
+    if (m_width && m_height)
         m_bmpUnderCaret = wxBitmap(m_width, m_height);
+    else
+        m_bmpUnderCaret = wxBitmap();
 #endif
     if (countVisible > 0)
     {