egl: check for NULL value like eglGetSyncAttribKHR does
Commit d1e1563b added a NULL check for eglGetSyncAttribKHR but eglGetSyncAttrib does not do this. Patch adds same check to happen with eglGetSyncAttrib.
Fixes crashes in (when exposing EGL 1.5): dEQP-EGL.functional.fence_sync.invalid.get_invalid_value
Signed-off-by: Tapani Pälli tapani.palli@intel.com