Tidy up overflows in obj_cmp.

While this isn't really an issue, don't use the a - b comparator pattern since
it doesn't account for overflows. (They'll also break silently if that field
ever becomes unsigned as it should be.)

Change-Id: I613d19df6e4a785efd4cffd46e8b03dbc95b98e2
Reviewed-on: https://boringssl-review.googlesource.com/4890
Reviewed-by: Adam Langley <agl@google.com>