diag: dci: check signed values for negativity

when comparing a signed value vs sizeof, make sure
it's >= 0 so that when promoted to signed for comparison
it won't lead to unexpected results.

Bug: 70528036
Change-Id: I99a1220598e67e3ce5c99d425e84c79639121838
Signed-off-by: Andrew Chant <achant@google.com>
(cherry picked from commit 7970e95b194b8e4da3c959e899c82c1402f29ea3)