pgsql: Fix buggy usage of vsnprintf in PL/Python by removing it

Log Message:
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Fix buggy usage of vsnprintf in PL/Python by removing it altogether, instead
relying on stringinfo.c.  This fixes a problem reported by Marko Kreen, but I
didn't use his patch, per subsequent discussion.

Tags:
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REL8_0_STABLE

Modified Files:
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    pgsql/src/pl/plpython:
        plpython.c (r1.58.4.6 -> r1.58.4.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.58.4.6&r2=1.58.4.7)