pgsql: Change exec_eval_simple_expr's param list allocation call from

Log Message:
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Change exec_eval_simple_expr's param list allocation call from
MemoryContextAllocZero back to MemoryContextAlloc, same as it was in 7.4.
The zeroing is unnecessary since all the meaningful fields are filled in
just below.  I had made it do that out of neatnik-ism, but some testing
with an example provided by Pavel Stehule showed that the zeroing was
accounting for about 5% of the runtime in a compute-intensive plpgsql
function.  That seems a bit high of a price for neatnik-ism...

Modified Files:
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    pgsql/src/pl/plpgsql/src:
        pl_exec.c (r1.126 -> r1.127)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.126&r2=1.127)