reset minSampleShading only with OpenGL Core 4.0+
reset minSampleShading only with OpenGL Core 4.0+

This fixes crashes seen with many tests when context state is
reset and gl.minSampleShading is NULL.

Affects: KHR-GL*

Components: OpenGL

VK-GL-CTS issue: 2827

Fixes: 327ee5674 ("Support GL 4.5 in various multisample tests")
Signed-off-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>
Change-Id: I4b67a30264d4bdb6de8a5f7e811ae8bb8ee8a696

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