device_limits: improve EnumerateDeviceExtensionProperties

Call down the next layer unless pLayerName is device_limits.
diff --git a/layers/device_limits.cpp b/layers/device_limits.cpp
index 90c7034..ba72127 100644
--- a/layers/device_limits.cpp
+++ b/layers/device_limits.cpp
@@ -670,13 +670,14 @@
 EnumerateDeviceExtensionProperties(VkPhysicalDevice physicalDevice,
                                    const char *pLayerName, uint32_t *pCount,
                                    VkExtensionProperties *pProperties) {
-    if (pLayerName == NULL) {
-        dispatch_key key = get_dispatch_key(physicalDevice);
-        layer_data *my_data = get_my_data_ptr(key, layer_data_map);
-        return my_data->instance_dispatch_table->EnumerateDeviceExtensionProperties(physicalDevice, NULL, pCount, pProperties);
-    } else {
+    if (pLayerName && !strcmp(pLayerName, global_layer.layerName))
         return util_GetExtensionProperties(0, nullptr, pCount, pProperties);
-    }
+
+    assert(physicalDevice);
+
+    dispatch_key key = get_dispatch_key(physicalDevice);
+    layer_data *my_data = get_my_data_ptr(key, layer_data_map);
+    return my_data->instance_dispatch_table->EnumerateDeviceExtensionProperties(physicalDevice, pLayerName, pCount, pProperties);
 }
 
 VKAPI_ATTR PFN_vkVoidFunction VKAPI_CALL
@@ -820,7 +821,8 @@
 VK_LAYER_EXPORT VKAPI_ATTR VkResult VKAPI_CALL vkEnumerateDeviceExtensionProperties(VkPhysicalDevice physicalDevice,
                                                                                     const char *pLayerName, uint32_t *pCount,
                                                                                     VkExtensionProperties *pProperties) {
-    return device_limits::EnumerateDeviceExtensionProperties(physicalDevice, pLayerName, pCount, pProperties);
+    // the layer command handles VK_NULL_HANDLE just fine
+    return device_limits::EnumerateDeviceExtensionProperties(VK_NULL_HANDLE, pLayerName, pCount, pProperties);
 }
 
 VK_LAYER_EXPORT VKAPI_ATTR PFN_vkVoidFunction VKAPI_CALL vkGetDeviceProcAddr(VkDevice dev, const char *funcName) {