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 [email protected], [email protected] Review URL: https://codereview.appspot.com/9193045 git-svn-id: http://angleproject.googlecode.com/svn/trunk@2207 736b8ea6-26fd-11df-bfd4-992fa37f6226