Fix uninitialized local variables in generated shader.
Fix uninitialized local variables in generated shader.

There is uninitialized variable which is used as texture sampler's
coordinate. The behavior is undefined according to spec.

Affects:

dEQP-GLES3.functional.shaders.random.*

Components: Framework, OpenGL
VK-GL-CTS issue: 2972

Change-Id: I3e68a1a2158d248b24c927c481ecba4001496b3f

KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS