PATCH: Fix wrong size argument to pg_strncasecmp

Hello,

Please find a one-liner patch in the attachment.

This patch fixes a size parameter of `pg_strncasecmp` which compared a
"string" literal with a variable by passing a size of 5 while the "string"
literal has 6 bytes.

This issue can be observed with the following query (where 'X' is any
character other than 'g' and null byte):

select json_to_tsvector('"abc"'::json, '"strinX"')

Before this patch this query returns the `'abc':1` result instead of
failing with the following error:

wrong flag in flag array: "strinX"

By the way, the `strncasecmp` usages around the fixed line could use
`strcasecmp` which doesn't accept the `size_t n` argument.

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Regards,
Disconnect3d