Almost bug in COPY FROM processing of GB18030 encoded input

Hi,

I happened to notice that when CopyReadLineText() calls mblen(), it
passes only the first byte of the multi-byte characters. However,
pg_gb18030_mblen() looks at the first and the second byte.
CopyReadLineText() always passes \0 as the second byte, so
pg_gb18030_mblen() will incorrectly report the length of 4-byte encoded
characters as 2.

It works out fine, though, because the second half of the 4-byte encoded
character always looks like another 2-byte encoded character, in
GB18030. CopyReadLineText() is looking for delimiter and escape
characters and newlines, and only single-byte characters are supported
for those, so treating a 4-byte character as two 2-byte characters is
harmless.

Attached is a patch to explain that in the comments. Grepping for
mblen(), I didn't find any other callers that used mblen() like that.

- Heikki