Fix user data handling when reading off end of push const
Fix user data handling when reading off end of push const

This fixes a compiler crash introduced in #650
68414fe49 Rationalized user data usage

The crash happened when a shader uses a larger push constant than the
size indicated in the pipeline state, which is illegal according to the
Vulkan spec.

Fixes: https://github.com/GPUOpen-Drivers/AMDVLK/issues/135
Change-Id: Iec7ef011e89ed199b34609896699881b502b59c4

googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc