avoid stack-smash when processing unusual commit

[jim at meyering.net (Jim Meyering)]

Jim Meyering wrote:
> Resending, now that the list is functioning again.
> FTR, I also sent to hjemli at gmail.com (no reply) and finally
> resorted to filing a BZ: http://bugzilla.redhat.com/820733
>
> Jim Meyering wrote:
>> This started when I noticed some cgit segfaults on savannah.gnu.org.
>> Finding the offending URL/commit and then constructing a stand-alone
>> reproducer were far more time-consuming than writing the actual patch.
>>
>> The problem arises with a commit like this, in which the user name
>> part of the "Author" field is empty:
>>
>>     $ git log -1
>>     commit 6f3f41d73393278f3ede68a2cb1e7a2a23fa3421
>>     Author: <T at h.or>
>>     Date:   Mon Apr 23 22:29:16 2012 +0200

Jason,
Thanks for continuing cgit maintenance.
Here's a patch I wrote months ago.
The patch is tiny, compared to the analysis, but the analysis
took enough effort that I've included it in the commit log.

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