gallivm: fix crash in emit_get_buffer_size

gallivm: fix crash in emit_get_buffer_size

Seems a bit odd we extract a value from a vector in the first place
(as we always extract the first element), but llvm asserts if using
a zero-vector instead of zero as the index element.
Fixes piglit crashes for example in arb_shader_storage_buffer_object-layout-std140-write-shader.

Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3886>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3886>