[memcached] Coverage build failure

Similar to a few other previous projects (#10717, #11081, #11010), `memcached` is failing to build coverage:
```
Step #5: [/corpus/fuzzer_proxy.zip]
Step #5: End-of-central-directory signature not found. Either this file is not
Step #5: a zipfile, or it constitutes one disk of a multi-part archive. In the
Step #5: latter case the central directory and zipfile comment will be found on
Step #5: the last disk(s) of this archive.
Step #5: unzip: cannot find zipfile directory in one of /corpus/fuzzer_proxy.zip or
Step #5: /corpus/fuzzer_proxy.zip.zip, and cannot find /corpus/fuzzer_proxy.zip.ZIP, period.
Step #5: Failed to unpack the corpus for fuzzer_proxy. This usually means that corpus backup for a particular fuzz target does not exist. If a fuzz target was added in the last 24 hours, please wait one more day. Otherwise, something is wrong with the fuzz target or the infrastructure, and corpus pruning task does not finish successfully.
```

The fuzzer itself appears to be [running](https://oss-fuzz.com/fuzzer-stats?project=memcached&fuzzer=libFuzzer_memcached_fuzzer_proxy&job=libfuzzer_asan_memcached&group_by=by-day), and I see a corpus in `memcached-corpus.clusterfuzz-external.appspot.com`. I also can't replicate this issue locally. Any ideas?

Start by comparing previous issues #10717, #11081, and #11010 with the memcached coverage build failure. Check the linked fuzzer stats and the memcached corpus referenced in the report, then trace why /corpus/fuzzer_proxy.zip cannot be unpacked; done means the coverage build completes and the corpus is restored successfully.

Written by the indexing model from the issue text.