ilo: clear writer pointer after unmapping
ilo: clear writer pointer after unmapping

It does not look like an issue now but it is good to be future proof.  Spotted
by Courtney Goeltzenleuchter.

Signed-off-by: Chia-I Wu <1c78ba4519acc7012ab6d28d2e5ef6630c8f0d2e@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler