It happens frequently that unrelated compiler errors lead to the
instantiation of {read_leb_tail} with an invalid {byte_index}, which
again triggers compiler errors.
This change fixes this, such that illegal instantiation just triggers a
static_assert and nothing more. This helps to spot the actual error.

Drive-by: Mark {Min} and {Max} constexpr, such that they can produce
constexpr results. Note that the result will only be constexpr if both
arguments are constexpr.

R=ahaas@chromium.org

Change-Id: I35d6865a7f569d72ace0debb045ac615cfd87d92
Reviewed-on: https://chromium-review.googlesource.com/518005Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45558}