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: I096fe1c9b4f0f398f3a638cd8311278987dfb7dc
Reviewed-on: https://chromium-review.googlesource.com/263404Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>