layers: add some asserts to spirv_inst_iter
layers: add some asserts to spirv_inst_iter

These allow us to more easily catch in debug:
- Bogus zero-length instructions
- Reads off the end of the instruction

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

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