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: fd01840c0bd3 ("glsl: add AoA support to subroutines")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100438