pgsql: Although I'd misdiagnosed the reason for the recent failures on

Log Message:
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Although I'd misdiagnosed the reason for the recent failures on
buildfarm member grebe, I see no reason to revert the 1-byte-header-friendly
changes I made in varlena.c. Instead, tweak the code a little bit to
get more advantage out of that.

Modified Files:
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pgsql/src/backend/utils/adt:
varlena.c (r1.158 -> r1.159)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c?r1=1.158&r2=1.159)