glsl: avoid stack smashing when there are too many attributes

glsl: avoid stack smashing when there are too many attributes

This fixes a crash in

dEQP-GLES3.functional.transform_feedback.array_element.separate.points.lowp_mat3x2

and likely others. The vertex shader has > 16 input variables (without
explicit locations), which causes us to index outside of the to_assign
array.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>