Cycles: Fix an AttributeErro exceptionr on missing object context

This happens when the properties panel is pinned to the material tab.

Patch by Ralf Hölzemer (aka cheleb), thanks!

Differential Revision: https://developer.blender.org/D1776

intern/cycles/blender/addon/ui.py

@@ -610,9 +610,9 @@ class Cycles_PT_context_material(CyclesButtonsPanel, Panel):
         ob = context.object
         slot = context.material_slot
         space = context.space_data
-        is_sortable = len(ob.material_slots) > 1
 
         if ob:
+            is_sortable = len(ob.material_slots) > 1
             rows = 1
             if (is_sortable):
                 rows = 4