Also fixes the uniform count upper limit in glGetActiveUniformsiv,
as well as an assert hit with used but unbound uniform buffer.

BUG=angleproject:507
BUG=angleproject:962

Change-Id: I263b14df41d4e45a67304c1d145646398721cf0a
Reviewed-on: https://chromium-review.googlesource.com/263412Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>