Common: Fix copy-construction of empty BITMAP_BASE

The bitmap pointer is not checked at copy construct. This is
an instant segfault if you copy an empty bitmap.

Fix the constructor and remove the expected test failure, from the
previous commit.

common/bitmap_base.cpp

@@ -59,8 +59,15 @@ BITMAP_BASE::BITMAP_BASE( const BITMAP_BASE& aSchBitmap )
     m_scale = aSchBitmap.m_scale;
     m_ppi   = aSchBitmap.m_ppi;
     m_pixelScaleFactor = aSchBitmap.m_pixelScaleFactor;
-    m_image = new wxImage( *aSchBitmap.m_image );
-    m_bitmap = new wxBitmap( *m_image );
+
+    m_image = nullptr;
+    m_bitmap = nullptr;
+
+    if( aSchBitmap.m_image )
+    {
+        m_image = new wxImage( *aSchBitmap.m_image );
+        m_bitmap = new wxBitmap( *m_image );
+    }
 }