DrawElements' range was of the form [start, end] while DrawArrays' was
of the form [start, end), which caused an out of bound array access in
the client vertex pointers.

This issue was detected while running angle_end2end_tests with
AddressSanitizer.

BUG=angleproject:1137

Change-Id: Id9abddf29eaf73bacfd08d1616a999be2fe616b8
Reviewed-on: https://chromium-review.googlesource.com/295122Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>