layers: Fix paramcheck struct array NULL indexing
layers: Fix paramcheck struct array NULL indexing

Fixes an issue where the generated parameter_validation code could
index a NULL pointer to an array of structs.

Change-Id: If78f1c97ea256911d857b0b8009b38e6ca6f2581

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools