glsl/ast: Fix assertion failure when GS input declared as non-array.

glsl/ast: Fix assertion failure when GS input declared as non-array.

Previously, if a geometry shader input was declared as a non-array, we
would flag the proper compiler error, but then before we got a chance
to report it to the client, handle_geometry_shader_input_decl() would
assertion fail.

With this patch, handle_geometry_shader_input_decl() ignores
non-arrays.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>