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>