Handle storage access on vulkan with dynamic rendering in pixel history

* When using dynamic rendering we were falling through if we didn't find the
  image as an attachment as we expected to, but in the case of direct stores we
  don't expect to find it so we should return early rather than crashing trying
  to look up a non-existent renderpass object.

renderdoc/driver/vulkan/vk_pixelhistory.cpp

@@ -1466,6 +1466,7 @@ protected:
           }
         }
       }
+      return 0;
     }
 
     const VulkanCreationInfo::RenderPass &rpInfo =