Fix UBSan error in spirv-dis fuzzer (#4517)
Fix UBSan error in spirv-dis fuzzer (#4517)

Pending a more general solution for constructing a target environment
based on the bytes of a test input, this change avoids a UBSan error
caused by the existing approach.

Fixes https://crbug.com/38087
pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools