Fix redundant memset after palloc0 in heap_form_minimal_tuple()

Hi Hackers,

While studying the other patch, I happened to notice this problem
in heap_form_minimal_tuple().

The current code does both palloc0() and memset(0)
in heap_form_minimal_tuple():
```
/*
* Allocate and zero the space needed.
*/
mem = palloc0(len + extra);
memset(mem, 0, extra);
tuple = (MinimalTuple) (mem + extra);
```

That looks like an oversight of a0942f4.

To fix the problem, my first impression was to delete the memset(). But
looking at a0942f4, I found a couple of other places that do the same
pattern: palloc(len+extra) then memset(0, extra), so I think the correct
fix should be changing the palloc0 to palloc.

Best regards,
Chao Li (Evan)
---------------------
HighGo Software Co., Ltd.
https://www.highgo.com/