Fix random crash using Eyedropper with Alt+click

This was a problem related to uninitialized memory in Render class.
The Render::renderLayer() member function was accessing to an invalid
m_previewImage pointer.

src/render/render.cpp

@@ -328,6 +328,9 @@ Render::Render()
   , m_bgType(BgType::TRANSPARENT)
   , m_bgCheckedSize(16, 16)
   , m_globalOpacity(255)
+  , m_selectedLayer(nullptr)
+  , m_selectedFrame(-1)
+  , m_previewImage(nullptr)
   , m_onionskinType(OnionskinType::NONE)
 {
 }