cmd/compile/internal/gc: correctly use internal call to error reporting

Fixes #13266.

Change-Id: I31da922e0599989e52acf346374c2077b157ebb7
Reviewed-on: https://go-review.googlesource.com/16971Reviewed-by: Chris Manghane <cmang@golang.org>

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

@@ -116,7 +116,7 @@ func (p *parser) syntax_error(msg string) {
 		msg = ", " + msg
 	default:
 		// plain error - we don't care about current token
-		Yyerror("syntax error: " + msg)
+		Yyerror("syntax error: %s", msg)
 		return
 	}
 
@@ -138,7 +138,7 @@ func (p *parser) syntax_error(msg string) {
 		tok = tokstring(p.tok)
 	}
 
-	Yyerror("syntax error: unexpected " + tok + msg)
+	Yyerror("syntax error: unexpected %s", tok + msg)
 }
 
 // Advance consumes tokens until it finds a token of the stoplist.
@@ -1669,7 +1669,7 @@ func (p *parser) sym() *Sym {
 		return nil
 
 	default:
-		p.syntax_error("")
+		p.syntax_error("expecting name")
 		p.advance()
 		return new(Sym)
 	}

test/fixedbugs/issue13266.go

+// 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.
+
+// Offending character % must not be interpreted as
+// start of format verb when emitting error message.
+
+package% // ERROR "unexpected %"