cmd/compile: prevent panic while formatting func(...T) with unknown T

cmd/compile: prevent panic while formatting func(...T) with unknown T

Compile:

package p

var f = func(...A)

Before this CL:

x.go:3:13: type %!v(PANIC=runtime error: invalid memory address or nil pointer dereference) is not an expression
x.go:3:17: undefined: A

After this CL:

x.go:3:13: type func(...<T>) is not an expression
x.go:3:17: undefined: A

Found with go-fuzz.

Fixes #20233

Change-Id: Ibb232b3954c4091071440eba48b44c4022a8083f
Reviewed-on: https://go-review.googlesource.com/42610
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>