cmd/compile: fix clear on slice with zero size elem

cmd/compile: fix clear on slice with zero size elem

Fixed #61127

Change-Id: If07b04ebcc98438c66f273c0c94bea1f230dc2e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/507535
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>