Fix minor copy & pasto in the int128 accumulator patch.

It's unlikely that using PG_GETARG_INT16 instead of PG_GETARG_INT32 in
this pace can cause actual problems, but this still should be fixed.

src/backend/utils/adt/numeric.c

@@ -3019,7 +3019,7 @@ int2_accum(PG_FUNCTION_ARGS)
 	if (!PG_ARGISNULL(1))
 	{
 #ifdef HAVE_INT128
-		do_int128_accum(state, (int128) PG_GETARG_INT32(1));
+		do_int128_accum(state, (int128) PG_GETARG_INT16(1));
 #else
 		Numeric		newval;