cmd/compile/internal/parser: don't crash after unexpected token

Added missing nil-check. We will get rid of the gcCompat corrections
shortly but it's still worthwhile having the new test case added.

Fixes #19056.

Change-Id: I35bd938a4d789058da15724e34c05e5e631ecad0
Reviewed-on: https://go-review.googlesource.com/36908
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -799,7 +799,7 @@ loop:
 				p.syntax_error("expecting name or (")
 				p.advance(_Semi, _Rparen)
 			}
-			if gcCompat {
+			if gcCompat && x != nil {
 				x.init(p)
 			}
 

test/fixedbugs/issue19056.go

@@ -0,0 +1,9 @@
+// errorcheck
+
+// Copyright 2017 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.
+
+package p
+
+var _ = ... . // ERROR "unexpected ..."