cmd/link: make Loader.SymName not crash

Even if not presented with a valid symbol, recover gracefully,
so that debug prints do not crash.

Change-Id: I06bbe4bec5f90b79b4830e772a7fc3d7c919df1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/312036


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>