Fix VK_INSTANCE_LAYERS bug when using vk_loader_settings.json

The issue was that copy of the VK_INSTANCE_LAYERS would be 'iterated' to
find the next path, but wouldn't be reset back to the beginning of the
stack allocation when going to the next iteration of the loop. By
creating a separate char* to hold the "current path" pointer, the start
of the allocation isn't lost.

Change-Id: Ic3f0b818bda2f0a017bc30cb1f315b245008e9f7

loader/settings.c

@@ -726,6 +726,7 @@ VkResult enable_correct_layers_from_settings(const struct loader_instance* inst,
     if (vk_instance_layers_env != NULL) {
         vk_instance_layers_env_len = strlen(vk_instance_layers_env) + 1;
         vk_instance_layers_env_copy = loader_stack_alloc(vk_instance_layers_env_len);
+        memset(vk_instance_layers_env_copy, 0, vk_instance_layers_env_len);
 
         loader_log(inst, VULKAN_LOADER_WARN_BIT | VULKAN_LOADER_LAYER_BIT, 0, "env var \'%s\' defined and adding layers: %s",
                    ENABLED_LAYERS_ENV, vk_instance_layers_env);
@@ -761,13 +762,14 @@ VkResult enable_correct_layers_from_settings(const struct loader_instance* inst,
             loader_strncpy(vk_instance_layers_env_copy, vk_instance_layers_env_len, vk_instance_layers_env,
                            vk_instance_layers_env_len);
 
-            while (vk_instance_layers_env_copy && *vk_instance_layers_env_copy) {
-                char* next = loader_get_next_path(vk_instance_layers_env_copy);
-                if (0 == strcmp(vk_instance_layers_env_copy, props->info.layerName)) {
+            char* instance_layers_env_iter = vk_instance_layers_env_copy;
+            while (instance_layers_env_iter && *instance_layers_env_iter) {
+                char* next = loader_get_next_path(instance_layers_env_iter);
+                if (0 == strcmp(instance_layers_env_iter, props->info.layerName)) {
                     enable_layer = true;
                     break;
                 }
-                vk_instance_layers_env_copy = next;
+                instance_layers_env_iter = next;
             }
         }