pgsql: In CREATE CONVERSION, test that the given function is a valid

Log Message:
-----------
In CREATE CONVERSION, test that the given function is a valid conversion
function for the specified source and destination encodings. We do that by
calling the function with an empty string. If it can't perform the requested
conversion, it will throw an error.

Backport to 7.4 - 8.3. Per bug report #4680 by Denis Afonin.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/commands:
        conversioncmds.c (r1.29 -> r1.29.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c?r1=1.29&r2=1.29.2.1)