cmd/compile: avoid panic when printing *<N>
cmd/compile: avoid panic when printing *<N>

When compiling the program:

package p

func _(){
	*;:=
}

Before:

x.go:4:3: syntax error: unexpected semicolon, expecting expression
x.go:4:4: non-name *%!v(PANIC=runtime error: invalid memory address or nil pointer dereference) on left side of :=
x.go:5:1: syntax error: unexpected }, expecting expression

After:

x.go:4:3: syntax error: unexpected semicolon, expecting expression
x.go:4:4: non-name *<N> on left side of :=
x.go:5:1: syntax error: unexpected }, expecting expression

No test because:

(1) we don't have a good mechanism to check for the
    absence of the string "PANIC" in an error message
(2) the string "*<N>", while better, is itself ugly enough
    that I don't want to actively check for it
(3) the bug isn't very important, the kind of thing only fuzzers encounter
(4) the fix is obvious and trivial

Fixes #20220

Change-Id: I35faa986b60b671414ee999d6264b06937f250e3
Reviewed-on: https://go-review.googlesource.com/42498
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

derekmarcotte/go,cloudflare/go,golang/go,josharian/go,akutz/go,vsekhar/elastic-go,nawawi/go,golang/go,polyverse-security/go,ImJasonH/go,golang/go,nawawi/go,Cofyc/go,codestation/go,codestation/go,cloudflare/go,codestation/go,josharian/go,josharian/go,ImJasonH/go,ImJasonH/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,derekmarcotte/go,golang/go,cloudflare/go,vsekhar/elastic-go,akutz/go,nawawi/go,codestation/go,josharian/go,vsekhar/elastic-go,derekmarcotte/go,codestation/go,josharian/go,nawawi/go,ImJasonH/go,Cofyc/go,ImJasonH/go,Cofyc/go,Cofyc/go,Cofyc/go,CAFxX/go,derekmarcotte/go,polyverse-security/go,polyverse-security/go,polyverse-security/go,derekmarcotte/go,golang/go,josharian/go,akutz/go,CAFxX/go,vsekhar/elastic-go,derekmarcotte/go,ImJasonH/go,nawawi/go,akutz/go,cloudflare/go,codestation/go,derekmarcotte/go,CAFxX/go,akutz/go,nawawi/go,ImJasonH/go,CAFxX/go,Cofyc/go,polyverse-security/go,akutz/go,Cofyc/go,golang/go,polyverse-security/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,vsekhar/elastic-go,derekmarcotte/go,polyverse-security/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,vsekhar/elastic-go,cloudflare/go,christopher-henderson/Go,codestation/go,vsekhar/elastic-go,CAFxX/go,codestation/go,cloudflare/go,golang/go,Cofyc/go,ImJasonH/go