This patch intends to fix a crash issue in the win-clang build of
ANGLE by adding the missing currentValueAttrib->binding assignment
in StateManager11.cpp::updateCurrentValueAttribs(). In D3D11 all
vertex attributes (VertexAttribute and its VertexBinding) should be
treated as a whole, so we should always ensure these two parts are
availabe.

This patch also add ASSERTs before using *translated->attrib and
*translated->binding to prevent a TranslatedAttribute object
having a null attrib or binding.

BUG=chromium:721783, angleproject:1593

Change-Id: Id11d6f1d4c37daabad2265e01ea38eb51046b9cb
Reviewed-on: https://chromium-review.googlesource.com/505928Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>