mesa: Avoid read of uninitialized variable

mesa: Avoid read of uninitialized variable

The is_color_attachement variable is later read when handling two
separate error cases, where only one of the cases results in the
variable being initialized.

This can be avoided by giving the variable a safe default value.

Coverity-Id: 1398631
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>