cmd/compile/internal/gc: fix internal compiler error on invalid declaration

Following an empty import, a declaration involving a ? symbol
generates an internal compiler error when the name of the
symbol (in newname function).

package a
import""
var?

go.go:2: import path is empty
go.go:3: internal compiler error: newname nil

Make sure dclname is not called when the symbol is nil.
The error message is now:

go.go:2: import path is empty
go.go:3: invalid declaration
go.go:4: syntax error: unexpected EOF

This CL was initially meant to be applied to the old parser,
and has been updated to apply to the new parser.

Fixes #11610

Change-Id: I75e07622fb3af1d104e3a38c89d9e128e3b94522
Reviewed-on: https://go-review.googlesource.com/15268
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -1738,6 +1738,18 @@ func (p *parser) new_name(sym *Sym) *Node {
 	return nil
 }
 
+func (p *parser) dcl_name(sym *Sym) *Node {
+	if trace && Debug['x'] != 0 {
+		defer p.trace("dcl_name")()
+	}
+
+	if sym == nil {
+		yyerrorl(int(prevlineno), "invalid declaration")
+		return nil
+	}
+	return dclname(sym)
+}
+
 func (p *parser) onew_name() *Node {
 	if trace && Debug['x'] != 0 {
 		defer p.trace("onew_name")()
@@ -2736,9 +2748,9 @@ func (p *parser) dcl_name_list() *NodeList {
 		defer p.trace("dcl_name_list")()
 	}
 
-	l := list1(dclname(p.sym()))
+	l := list1(p.dcl_name(p.sym()))
 	for p.got(',') {
-		l = list(l, dclname(p.sym()))
+		l = list(l, p.dcl_name(p.sym()))
 	}
 	return l
 }

test/fixedbugs/issue11610.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test an internal compiler error on ? symbol in declaration
+// following an empty import.
+
+package a
+import""  // ERROR "import path is empty"
+var?      // ERROR "invalid declaration"
+
+var x int // ERROR "unexpected var"
+
+func main() {
+}