Fix ubsan reports in nir/radv

Addresses various issues caught while running vkglcts with ubsan enabled:

• Unaligned memory access
• Signed multiplication overflows
• memcpy(x, NULL, 0)
• Out-of-bounds array access

I also sneaked in a cleanup of related code via ffabca10 , let me know if I should move that to a separate MR.

Changes have been verified via vk-cts-image and had no impact on conformance or shader code gen.