pgsql-server: Fix silly thinko in ALTER COLUMN TYPE.

Log Message:
-----------
Fix silly thinko in ALTER COLUMN TYPE. Check for finding expected
dependency was looking at wrong columns and so would always fail.
Was not exposed by regression tests because we are only testing cases
involving built-in (pinned) types and so no actual dependency entry
exists to be removed.

Modified Files:
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pgsql-server/src/backend/commands:
tablecmds.c (r1.123 -> r1.124)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/tablecmds.c.diff?r1=1.123&r2=1.124)