Change exec_eval_simple_expr's param list allocation call from

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...