gallivm: fix bogus assert in get_indirect_index

0 is a valid value as max index, and the code handles it fine. This isn't
commonly seen, as it will only happen with array declarations of size 1.
Fixes piglit tests/shaders/complex-loop-analysis-bug.shader_test

Fixes: a3c898dc "gallivm: fix improper clamping of vertex index when fetching gs inputs"
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110441



Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit 88e0bbf2)