pgsql: Rearrange use of plpgsql_add_initdatums() so that only the

Log Message:
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Rearrange use of plpgsql_add_initdatums() so that only the parsing of a
DECLARE section needs to know about it. Formerly, everyplace besides DECLARE
that created variables needed to do "plpgsql_add_initdatums(NULL)" to prevent
those variables from being sucked up as part of a subsequent DECLARE block.
This is obviously error-prone, and in fact the SQLSTATE/SQLERRM patch had
failed to do it for those two variables, leading to the bug recently exhibited
by Asif Ali Rehman: a DECLARE within an exception handler tried to reinitialize
SQLERRM.

Although the SQLSTATE/SQLERRM patch isn't in any pre-8.1 branches, and so
I can't point to a demonstrable failure there, it seems wise to back-patch
this into the older branches anyway, just to keep the logic similar to HEAD.

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/src/pl/plpgsql/src:
gram.y (r1.64.4.4 -> r1.64.4.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.64.4.4&r2=1.64.4.5)
pl_comp.c (r1.83 -> r1.83.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c.diff?r1=1.83&r2=1.83.4.1)