Fix some egl robustness tests

Fix some egl robustness tests

Because the values of color were initially undefined an optimised
compiler can simply set all components of the vector to the value
being assigned to color[u_index] and eventually eliminate color and
u_index completely.

To avoid this we initialise color.

Affects:
dEQP-EGL.functional.robustness.reset_context.shaders*

Components: OpenGL
VK-GL-CTS issue: 2349

Change-Id: I6eb929eec07b039afb247427af91aba65dbade9b
(cherry picked from commit 4392facc87c2da80b5065f8663b9303fb1113e16)