Fix coredump in creating distributed table

[ljcui]

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Changelog category (leave one):

• Bug Fix (user-visible misbehaviour in official stable or prestable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Fix the coredump in the creation of distributed tables, when the parameters passed in are wrong

Detailed description / Documentation draft:

sql like this：
create table td1 engine = Distributed(test_shard_localhost, currentDatabase(), 't', xxHash32(val), default) as t;
If the fifth parameter default is not wrapped in quotation marks, a coredump will be generated.

It's okay to write like this:
create table td1 engine = Distributed(cluster_2shards_2replicas, currentDatabase(), 't', xxHash32(val), 'default') as t;

[vdimir]

Great catch!