ensure_gn_version.py: don't die if existing gn is very old

Large biesect jumps can land in times where gn had a different
version string format -- just a plain number like "564007". Check
that the version regex matched before using the match so this case
is not a surprising failure.

Bug: 944367
Change-Id: I1b2cc2b269084d92d871d53daafc5c5e63c1eb9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710562
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Tomasz Śniatowski <tsniatowski@vewd.com>
Cr-Original-Commit-Position: refs/heads/master@{#679252}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8c47145dceeebf9686394acb3b3e64292b006fca
1 file changed