printer: check if specs exist before accessing them in genDecl printer

Checks that specs exist before attempting to access the first element in genDecl printer.

Change-Id: I3619bcabf6fec64c88b7a10cdb7be355e9e40559
GitHub-Last-Rev: 54cf699c969cb88cfd269f847a2bb4f25916be94
GitHub-Pull-Request: golang/go#30823
Reviewed-on: https://go-review.googlesource.com/c/go/+/167390Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/printer/nodes.go

@@ -1568,7 +1568,7 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 		}
 		p.print(d.Rparen, token.RPAREN)
 
-	} else {
+	} else if len(d.Specs) > 0 {
 		// single declaration
 		p.spec(d.Specs[0], 1, true)
 	}