Clamped negative index access.

Fixed error that allowed negative index for accessing
vector, matrix, and array. Now we report compile error
and clamp the index to 0.

Re-arranged code around it to handle negative index
at the one location.

BUG=crbug.com/239411
TEST=bug test case
R=aedla@chromium.org, kbr@chromium.org

Review URL: https://codereview.appspot.com/9193045

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2207 736b8ea6-26fd-11df-bfd4-992fa37f6226