Do less work in zeroing out VMArray

We don't need to go up all the way to ssize, just until the old end
of the stuff with data. The rest is already cleared from back when
we resized ssize.

(O = Old cell; N = new cell; G = garbage left over data)
|--|OO|OO|--|
|OO|OO|GG|--| <- conditional moves the old stuff and zeros the trash
|NN|NN|--|--|

src/6model/reprs/VMArray.c

@@ -317,7 +317,7 @@ static void set_size_internal(MVMThreadContext *tc, MVMArrayBody *body, MVMuint6
                 elems * repr_data->elem_size);
         body->start = 0;
         /* fill out any unused slots with NULL pointers or zero values */
-        elems = zero_slots(tc, body, elems, ssize, repr_data->slot_type);
+        elems = zero_slots(tc, body, elems, start+elems, repr_data->slot_type);
     }
     else if (n < elems) {
         /* we're downsizing; clear off extra slots */