fix forgotten return in case of invalid line format from memcache ascii client

[sivukhin]

I ran govanish linter (it still in the WIP phase) against godropbox repo and it found one suspicious place:

2023/12/24 17:03:42 it seems like your code vanished from compiled binary: func=[Stat], file=[/home/sivukhin/code/godropbox/repo/memcache/raw_ascii_client.go], line=[675], snippet:
	NewStatErrorResponse(err, shardEntries)

Obviously, this is a bug - return statement were forgotten.

This PR fixes this bug.

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