selftests/bpf: No non-NULL inference from an imprecise zero register

Check that a register-form NULL check does not lift PTR_MAYBE_NULL on
a path where the compared register is non-zero. W/o the previous patch
the program is accepted.

Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260904083325.2083493-8-eddyz87@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>