cmd/link: skip zero values in fingerprint check

Normally, packages are loaded in dependency order, and if a
Library object is not nil, it is already loaded with the actual
fingerprint. In shared build mode, however, packages may be added
not in dependency order (e.g. go install -buildmode=shared std
adds all std packages before loading them), and it is possible
that a Library's fingerprint is not yet loaded. Skip the check
in this case (when the fingerprint is the zero value).

Fixes #39777.

Change-Id: I66208e92bf687c8778963ba8e33e9bd948f82f3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/239517


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>