bpftool: btf: Validate root_type_ids early

Handle invalid root_type_ids early, as an invalid ID will cause dumpers
to half-emit valid boilerplate and then bail with an unclean exit. This
is ugly and possibly confusing for users, so preemptively handle the
common error case before any dumping begins.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/33e09a08a6072f8381cb976218a009709309b7e1.1734119028.git.dxu@dxuuu.xyz