bpf: Add a possibly-zero-sized read test

This patch adds a test for the condition that the previous patch mucked
with - illegal zero-sized helper memory access. As opposed to existing
tests, this new one uses a size whose lower bound is zero, as opposed to
a known-zero one.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231221232225.568730-3-andreimatei1@gmail.com