glsl: Catch subscripted calls to undeclared subroutines
generate_array_index fails to check whether the target of a subroutine call exists in the AST, potentially passing around null ir_rvalue pointers eventuating in abort/segfault. Fixes: fd01840c ("glsl: add AoA support to subroutines") Reviewed-by:Timothy Arceri <tarceri@itsqueeze.com> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100438