Don't access arrays out-of-bounds in robustness2 tests

Don't access arrays out-of-bounds in robustness2 tests

The SPIR-V spec says that indexing a pointer to an array with an
out-of-bounds index leads to undefined behavior. While
VK_EXT_robustness2 allows you to access outside the range of the
underlying descriptor, it doesn't change this fact, so we should be
careful to only access inside of the shader-declared array when testing
UBO's. For SSBO's we switch to using an unsized array to keep test
coverage of negative indices.

Affects:
dEQP-VK.robustness.robustness2.*

VK-GL-CTS Issue: 2649
Components: Vulkan

Change-Id: I60a4b80749278f4d81defdbe2abe116a1dcfa159