loader: Fix handling of JSON
loader: Fix handling of JSON

If a JSON layer manifest file says it contained device extensions
but didn't provide a list, or the list was malformed, it caused
issues.

Change-Id: I94b97b190aca35f9464918eb7195486566becaa0

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers