git_superproject: fix AttributeError in Superproject logging

Ensure _git_event_log is initialized before use in _LogMessage. This
avoids crashes when _git_event_log is accessed before it's set, such as
during repo info.

Bug: 435317391
Change-Id: I3adc32d6a9377558e852bbb43f9cf82041fcf1bc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/498521
Commit-Queue: Gavin Mak <gavinmak@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>

git_superproject.py

@@ -190,6 +190,7 @@ class Superproject:
         message = f"{self._LogMessagePrefix()} {fmt.format(*inputs)}"
         if self._print_messages:
             print(message, file=sys.stderr)
+        if self._git_event_log:
             self._git_event_log.ErrorEvent(message, fmt)
 
     def _LogMessagePrefix(self):