Fix DROP OPERATOR FAMILY IF EXISTS.

Robert Haas <rhaas@postgresql.org>

Commit: 980261929f2b8c40d6be1979ff81c943cad907b3
Author: Robert Haas <rhaas@postgresql.org>
Date: 2011-10-21T13:12:23Z
Releases: 9.2.0
Fix DROP OPERATOR FAMILY IF EXISTS.

Essentially, the "IF EXISTS" portion was being ignored, and an error
thrown anyway if the opfamily did not exist.

I broke this in commit fd1843ff8979c0461fb3f1a9eab61140c977e32d; so
backpatch to 9.1.X.

Report and diagnosis by KaiGai Kohei.

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