(1): "substart == ex->ex_end" and "subend == ex->ex_start"
are completely legal parameters for extent_alloc_subregion()
(2): "(subend - substart) + 1" can cause an overflow if the whole
numeric range is covered by the extent.
Submitted by Matthias Drochner <drochner@zelz26.zel.kfa-juelich.de>
in PR #3119.