spec: handle nullptr vkGetInstanceProcAddr

tools/spec.py

@@ -880,6 +880,18 @@ def write_dispatch(dst: TextIO, q: List[Type], reg: Registry):
             return err;
         """,
 
+        'vkGetInstanceProcAddr': """
+            if (instance == VK_NULL_HANDLE) {
+                return (PFN_vkVoidFunction)cruft::vk::load::v_table->vk_icdGetInstanceProcAddr (instance, pName);
+            }
+        
+            auto const entry = reinterpret_cast<indirect<VkInstance,cruft::vk::load::instance_table> const*> (instance);
+
+            return (entry->table.vkGetInstanceProcAddr)(
+                entry->handle, pName
+            );
+        """,
+
         'vkCreateDevice': """
             (void)physicalDevice;
             (void)pCreateInfo;