loader_add_to_ext_list() returns VkResult and documents that
VK_ERROR_OUT_OF_HOST_MEMORY signals allocation failure, but two call
sites discarded the return value entirely:

1. loader_read_layer_json() – when appending an instance extension from
   a layer's JSON manifest the OOM path silently drops the extension and
   continues.  The device-extension block directly below it correctly
   checks the analogous loader_add_to_dev_ext_list() return value;
   bring instance-extension parsing in line with it.

2. terminator_EnumerateInstanceExtensionProperties() – when merging
   implicit-layer extensions into the local list the return value was
   ignored.  On allocation failure the function continues with an
   incomplete list and returns VK_SUCCESS to the application, violating
   the Vulkan spec guarantee that a successful count query returns the
   full set of available extensions.

Capture the return value in both sites and propagate
VK_ERROR_OUT_OF_HOST_MEMORY via the existing 'goto out' paths.

Fixes: silent OOM swallowing in loader_read_layer_json
Fixes: silent OOM swallowing in terminator_EnumerateInstanceExtensionProperties
Reviewed-by: (pending)