EbuildMetadataPhase: make _unregister idempotent (bug 711362)

Make the _unregister method check if self._files is None, as
necessary for idempotency. This will prevent the AttributeError
reported in bug 711362, which was triggered by recent changes
in callback scheduling order.

Closes: https://bugs.gentoo.org/711362
Signed-off-by: Zac Medico <zmedico@gentoo.org>
(cherry picked from commit 0b78b4a90657ef582a68258f27eaf521dfb42038)

BUG=b:238219234
TEST=cros_portage_upgrade --upgrade --board reven app-editors/vim

Change-Id: I50c3faec5058c1aa04a365b7d48ebad859f9d70a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3749477
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Matt Turner <msturner@google.com>
Commit-Queue: Matt Turner <msturner@google.com>
Tested-by: Matt Turner <msturner@google.com>
1 file changed