dwarflint: Don't assert in check_debug_info can trigger an assert.

dwarflint: Don't assert in check_debug_info can trigger an assert.

Instead of asserting a form must exist, let form_class () return
max_dw_class, like ambiguous_class () does. max_dw_class is already
used as sentinel in read_die_chain () in case the attribute is unknown.
This lets things work out for the low level check_debug_info check and
then makes the higher-level checks complain about the unexpected form
instead. Added an example binary and test dwarflint/tests/run-upper.sh.