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 <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/fmt.go

@@ -1509,16 +1509,10 @@ func (n *Node) exprfmt(s fmt.State, prec int, mode fmtMode) {
 		}
 		mode.Fprintf(s, "make(%v)", n.Type)
 
+	case OPLUS, OMINUS, OADDR, OCOM, OIND, ONOT, ORECV:
 		// Unary
-	case OPLUS,
-		OMINUS,
-		OADDR,
-		OCOM,
-		OIND,
-		ONOT,
-		ORECV:
 		mode.Fprintf(s, "%#v", n.Op)
-		if n.Left.Op == n.Op {
+		if n.Left != nil && n.Left.Op == n.Op {
 			fmt.Fprint(s, " ")
 		}
 		n.Left.exprfmt(s, nprec+1, mode)