Fix warnings reported by clang 13

Fix warnings reported by clang 13

This commit fixes code that was producing warnings when built using
clang 13. They are mostly about unused variables and null pointer
substractions, but the new code should be semantically the same.

No test results should be affected by these changes.

Components: Framework, Vulkan, OpenGL
VK-GL-CTS issue: 3147

Change-Id: I0eb87c17270a610c8b29ec27ae8e10009053f8e1