softpipe: Fix the const buffer overflow check.
The size is in bytes, pos is a dword index.
Fixes these asan failures (not tested in CI since we run a fraction):
dEQP-GLES31.functional.shaders.opaque_type_indexing.ubo.const_expression_compute,Crash dEQP-GLES31.functional.shaders.opaque_type_indexing.ubo.const_literal_compute,Crash