WP_Comment_Query can generate negative offset in LIMIT clause

[birgire]

The paged argument in WP_Comment_Query was introduced in #38268

There's a problem with the paged as 0 case.

Example:

When the paged is 0 and number is 2, WP_Comment_Query generates the LIMIT clause:

LIMIT -2, 2

instead of

LIMIT 0, 2

According to the MySQL documentation, the offset in the LIMIT clause shouldn't be negative:

​https://dev.mysql.com/doc/refman/5.7/en/select.html

I think we should handle it in the same way as in WP_Query, where the 0 case is treated as the default, that is 1.

[birgire]

Here we treat paged 0 as the default value, i.e. 1.

We try to show it explicitly in the code, so it's more obvious to the reader, similar how it's done in WP_Query.

Included a test for this case.