sync: uninitialized variable on mirror sync failure

When repo sync fails, if the workspace is a mirror, an uninitialized
variable is referenced.

Bug: crbug.com/gerrit/16356
Change-Id: I1dba9f92319b9cbfd18460327560a395c88a089f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/349654
Reviewed-by: Sam Saccone <samccone@google.com>
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 8dc2214..fe63b48 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -1277,6 +1277,7 @@
 
     err_network_sync = False
     err_update_projects = False
+    err_update_linkfiles = False
 
     self._fetch_times = _FetchTimes(manifest)
     if not opt.local_only: