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>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 99cbec0a)